White Mountains (NYSE: WTM) renews Ark Outrigger Re sidecar with $70M for 2026

Rhea-AI Filing Summary

White Mountains Insurance Group, Ltd. reported that its specialty property and casualty subsidiary, Ark Insurance Holdings Limited, has renewed Outrigger Re Ltd., a reinsurance sidecar, for business incepting during calendar year 2026 on terms similar to 2025. Outrigger has again entered into a quota share with Ark to provide collateralized reinsurance protection on a portion of Ark Bermuda’s global property catastrophe portfolio.

For the 2026 underwriting year, Outrigger deployed $70 million of total investor capital, all provided by third-party investors and excluding White Mountains. The filing notes that Outrigger’s size was reduced compared with the prior year because Ark increased its use of traditional quota share reinsurance for 2026.

FALSE0000776867Bermuda1-899394-270845503755☐☐☐☐☐00007768672026-01-072026-01-07

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

January 7, 2026

Date of Report (Date of earliest event reported)

WHITE MOUNTAINS INSURANCE GROUP, LTD.

(Exact name of registrant as specified in its charter)

Bermuda (State or other jurisdiction of  incorporation or organization) 1-8993 (Commission file number) 94-2708455 (I.R.S. Employer Identification No.)

23 South Main Street, Suite 3B, Hanover, New Hampshire 03755

(Address of principal executive offices)

(603) 640-2200

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Title of each class

Trading Symbol(s)

Name of each exchange on which registered

Common Shares, par value $1.00 per share

WTM

New York Stock Exchange

WTM.BH

Bermuda Stock Exchange

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933(§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).                                     Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.      o

ITEM 8.01 Other Events.

White Mountains Insurance Group, Ltd. (NYSE: WTM) (“White Mountains”) announced today that Ark Insurance Holdings Limited (“Ark”), its specialty property and casualty reinsurance and insurance subsidiary, has renewed Outrigger Re Ltd. ("Outrigger") for business incepted during calendar year 2026. The renewal is on similar terms as were in effect for calendar year 2025.

Outrigger, a reinsurance sidecar, has again entered into a quota share with Ark to provide collateralized reinsurance protection on a portion of Ark Bermuda's global property catastrophe portfolio.

Outrigger deployed $70 million of total investor capital for the 2026 underwriting year. This capital was provided entirely by third-party investors excluding White Mountains. The reduction in Outrigger’s size was driven by an increase in Ark’s use of traditional quota share reinsurance for 2026.

2

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

WHITE MOUNTAINS INSURANCE GROUP, LTD.

Date: January 7, 2026

By:

/s/   MICHAELA J. HILDRETH            Michaela J. Hildreth       Managing Director and       Chief Accounting Officer

3

FAQ

What did White Mountains Insurance Group (WTM) announce about Ark and Outrigger Re for 2026?

White Mountains Insurance Group announced that its subsidiary Ark Insurance Holdings Limited renewed Outrigger Re Ltd. for business incepting during calendar year 2026, on similar terms as in 2025, to provide collateralized reinsurance protection on part of Ark Bermuda’s global property catastrophe portfolio.

How much capital is Outrigger Re deploying for the 2026 underwriting year for WTM’s Ark?

For the 2026 underwriting year, Outrigger Re is deploying $70 million of total investor capital to support a quota share of Ark Bermuda’s global property catastrophe portfolio.

Who provides the investor capital for Outrigger Re’s 2026 underwriting year related to WTM?

The $70 million of capital deployed by Outrigger Re for the 2026 underwriting year is provided entirely by third-party investors and specifically excludes White Mountains itself.

Why was Outrigger Re’s size reduced for the 2026 underwriting year at Ark?

The filing states that the reduction in Outrigger Re’s size for the 2026 underwriting year was driven by an increase in Ark’s use of traditional quota share reinsurance for 2026.

What role does Outrigger Re play in White Mountains’ Ark Bermuda operations?

Outrigger Re acts as a reinsurance sidecar that has entered into a quota share with Ark, providing collateralized reinsurance protection on a portion of Ark Bermuda’s global property catastrophe portfolio.

Is the 2026 Outrigger Re arrangement for WTM similar to the 2025 structure?

Yes. The renewal of Outrigger Re for business incepting during calendar year 2026 is described as being on similar terms as were in effect for calendar year 2025.