Investment Securities (Tables)

12 Months Ended

Dec. 31, 2025

Investments, Debt and Equity Securities [Abstract]

Schedule of pre-tax net investment income

The following table presents pre-tax net investment income for the years ended December 31, 2025, 2024 and 2023:

	Year Ended December 31,
Millions	2025		2024		2023
Fixed maturity investments	$	111.8	$	93.2	$	64.4
Short-term investments	47.8		56.6		52.0
Common equity securities	.6		2.5		4.1
Other long-term investments	90.4		77.9		76.1
Total investment income	250.6		230.2		196.6
Third-party investment expenses	(2.6)		(2.8)		(2.4)
Net investment income, pre-tax	$	248.0	$	227.4	$	194.2

Schedule of net realized and unrealized investment gains and losses

The following table presents net realized and unrealized investment gains (losses) for the years ended December 31, 2025, 2024 and 2023:

	Year Ended December 31,
Millions	2025		2024		2023
Realized investment gains (losses)
Fixed maturity investments	$	.8	$	(10.1)	$	(4.7)
Short-term investments	.5		(1.2)		(.2)
Common equity securities	67.7		3.5		29.5
Investment in MediaAlpha	—		91.2		—
Other long-term investments	42.5		34.6		72.2
Net realized investment gains (losses)	111.5		118.0		96.8
Unrealized investment gains (losses)
Fixed maturity investments	53.9		3.9		66.1
Short-term investments	.5		(.8)		1.9
Common equity securities	(14.2)		58.1		47.1
Investment in MediaAlpha	29.6		(53.2)		27.1
Other long-term investments	180.2		58.9		195.2
Net unrealized investment gains (losses)	250.0		66.9		337.4
Net realized and unrealized investment gains (losses) (1)	$	361.5	$	184.9	$	434.2
Fixed maturity and short-term investments
Net realized and unrealized investment gains (losses)	$	55.7	$	(8.2)	$	63.1
Less: net realized and unrealized gains (losses) on investment    securities sold during the period	5.7		.9		4.7
Net unrealized investment gains (losses) recognized during the    period on investment securities held at the end of the period	$	50.0	$	(9.1)	$	58.4
Common equity securities and investment in MediaAlpha
Net realized and unrealized investment gains (losses) on    common equity securities	$	53.5	$	61.6	$	76.6
Net realized and unrealized investment gains (losses) from    investment in MediaAlpha	29.6		38.0		27.1
Total net realized and unrealized investment gains (losses)	83.1		99.6		103.7
Less: net realized and unrealized gains (losses) on investment    securities sold during the period	6.4		35.4		28.4
Net unrealized investment gains (losses) recognized during the    period on investment securities held at the end of the period	$	76.7	$	64.2	$	75.3

(1) For 2025, 2024 and 2023, includes $37.1, $(31.5) and $13.6 of net realized and unrealized investment gains (losses) related to foreign currency exchange.

The following table presents total net unrealized gains (losses) attributable to Level 3 investments for the years ended December 31, 2025, 2024 and 2023 for investments still held at the end of the period:

	Year Ended December 31,
Millions	2025		2024		2023
Total net unrealized investment gains recognized during the period on    Level 3 investments held at the end of period	$	127.7	$	57.2	$	133.9

Schedule of equity securities and other long-term investments

The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of White Mountains’s fixed maturity investments as of December 31, 2025 and 2024:

	December 31, 2025
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
U.S. Government and agency obligations	$	459.7	$	1.3	$	(.7)	$	—	$	460.3
Debt securities issued by corporations	1,534.9		13.5		(11.5)		.8		1,537.7
Mortgage and asset-backed securities	405.1		4.2		(16.3)		—		393.0
Collateralized loan obligations	343.9		.8		(.1)		2.4		347.0
Foreign government and agency obligations	31.2		—		(.1)		1.4		32.5
Total fixed maturity investments	$	2,774.8	$	19.8	$	(28.7)	$	4.6	$	2,770.5

	December 31, 2024
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
U.S. Government and agency obligations	$	461.8	$	1.1	$	(.8)	$	—	$	462.1
Debt securities issued by corporations	1,444.5		4.6		(31.4)		(3.5)		1,414.2
Municipal obligations	3.2		—		—		—		3.2
Mortgage and asset-backed securities	400.2		.2		(26.5)		—		373.9
Collateralized loan obligations	237.3		1.2		(.2)		(1.6)		236.7
Foreign government and agency obligations	22.2		—		(.1)		(.6)		21.5
Total fixed maturity investments	$	2,569.2	$	7.1	$	(59.0)	$	(5.7)	$	2,511.6

The following tables present the cost or amortized cost, gross unrealized investment gains (losses), net foreign currency gains (losses) and carrying value of common equity securities, White Mountains’s investment in MediaAlpha and other long-term investments as of December 31, 2025 and 2024:

	December 31, 2025
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
Common equity securities	$	384.3	$	99.9	$	—	$	(1.2)	$	483.0
Investment in MediaAlpha	$	59.2	$	172.0	$	—	$	—	$	231.2
Other long-term investments	$	2,378.8	$	712.6	$	(123.1)	$	(9.8)	$	2,958.5

	December 31, 2024
Millions	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Net Foreign Currency Gains (Losses)		Carrying Value
Common equity securities	$	537.0	$	120.4	$	—	$	(7.4)	$	650.0
Investment in MediaAlpha	$	59.2	$	142.4	$	—	$	—	$	201.6
Other long-term investments	$	1,748.7	$	546.4	$	(116.7)	$	(28.2)	$	2,150.2

Schedule of contractual maturities of fixed maturities

The following table presents the cost or amortized cost and carrying value of White Mountains’s fixed maturity investments by contractual maturity as of December 31, 2025 and 2024. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without penalties.

	December 31, 2025				December 31, 2024
Millions	Cost or Amortized Cost		Carrying Value		Cost or Amortized Cost		Carrying Value
Due in one year or less	$	455.3	$	455.2	$	205.8	$	203.9
Due after one year through five years	1,349.8		1,354.1		1,494.5		1,478.7
Due after five years through ten years	199.0		199.3		206.1		193.2
Due after ten years	21.7		21.9		25.3		25.2
Mortgage and asset-backed securities and    collateralized loan obligations	749.0		740.0		637.5		610.6
Total fixed maturity investments	$	2,774.8	$	2,770.5	$	2,569.2	$	2,511.6

Schedule of fair value measurements for investment securities by level

The following tables present White Mountains’s fair value measurements for investments as of December 31, 2025 and 2024 by level. See Note 1 — “Basis of Presentation and Significant Accounting Policies.” The major security types were based on the legal form of the securities. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments and agencies, municipalities, entities issuing mortgage and asset-backed securities or entities issuing collateralized loan obligations vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated this asset class into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Bloomberg U.S. Intermediate Aggregate Index.

	December 31, 2025
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	460.3	$	460.3	$	—	$	—
Debt securities issued by corporations:
Financials	531.5		—		531.5		—
Consumer	306.2		—		306.2		—
Industrial	150.0		—		150.0		—
Healthcare	146.1		—		146.1		—
Technology	98.4		—		98.4		—
Utilities	90.4		—		90.4		—
Materials	78.7		—		78.7		—
Communications	76.3		—		76.3		—
Energy	60.1		—		60.1		—
Total debt securities issued by corporations	1,537.7		—		1,537.7		—
Mortgage and asset-backed securities	393.0		—		393.0		—
Collateralized loan obligations	347.0		—		347.0		—
Foreign government and agency obligations	32.5		—		32.5		—
Total fixed maturity investments	2,770.5		460.3		2,310.2		—
Short-term investments	1,881.7		1,871.7		10.0		—
Common equity securities:
Exchange-traded funds	30.7		30.7		—		—
Other (1)	452.3		—		452.3		—
Total common equity securities	483.0		30.7		452.3		—
Investment in MediaAlpha	231.2		231.2		—		—
Other long-term investments	1,848.1		—		33.7		1,814.4
Other long-term investments — NAV (2)	1,110.4		—		—		—
Total other long-term investments	2,958.5		—		33.7		1,814.4
Total investments	$	8,324.9	$	2,593.9	$	2,806.2	$	1,814.4

(1) Consists of investments in listed funds that predominantly invest in international equities.

(2) Consists of investments in the BroadStreet SPV, private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using NAV as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy.

	December 31, 2024
Millions	Fair Value		Level 1		Level 2		Level 3
Fixed maturity investments:
U.S. Government and agency obligations	$	462.1	$	461.3	$	.8	$	—
Debt securities issued by corporations:
Financials	466.7		—		466.7		—
Consumer	255.2		—		255.2		—
Industrial	164.4		—		164.4		—
Healthcare	153.1		—		153.1		—
Technology	113.8		—		113.8		—
Utilities	60.9		—		60.9		—
Materials	65.7		—		65.7		—
Communications	71.6		—		71.6		—
Energy	62.8		—		62.8		—
Total debt securities issued by corporations	1,414.2		—		1,414.2		—
Municipal obligations	3.2		—		3.2		—
Mortgage and asset-backed securities	373.9		—		373.9		—
Collateralized loan obligations	236.7		—		236.7		—
Foreign government and agency obligations	21.5		—		21.5		—
Total fixed maturity investments	2,511.6		461.3		2,050.3		—
Short-term investments	964.2		951.1		13.1		—
Common equity securities:
Exchange-traded funds	224.6		224.6		—		—
Other (1)	425.4		—		425.4		—
Total common equity securities	650.0		224.6		425.4		—
Investment in MediaAlpha	201.6		201.6		—		—
Other long-term investments	1,286.2		—		23.5		1,262.7
Other long-term investments — NAV (2)	864.0		—		—		—
Total other long-term investments	2,150.2		—		23.5		1,262.7
Total investments	$	6,477.6	$	1,838.6	$	2,512.3	$	1,262.7

(1) Consists of investments in listed funds that predominantly invest in international equities.

(2) Consists of investments in private equity funds and hedge funds, a bank loan fund, Lloyd’s trust deposits and ILS funds for which fair value is measured using NAV as a practical expedient. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy.

Schedule of credit ratings of debt securities issued

The following table presents the fair values for credit ratings of debt securities issued by corporations held in White Mountains’s investment portfolio as of December 31, 2025 and 2024:

	December 31,
Millions	2025		2024
AAA	$	7.8	$	14.6
AA	89.9		91.2
A	711.7		607.2
BBB	716.9		688.7
BB	3.2		5.4
Other	8.2		7.1
Debt securities issued by corporations (1)	$	1,537.7	$	1,414.2

(1) Credit ratings are based upon issuer credit ratings provided by Standard & Poor’s Financial Services LLC (“Standard & Poor’s”), or if unrated by Standard & Poor’s, long-term obligation ratings provided by Moody’s Investor Service, Inc.

Schedule of mortgage and asset-backed securities

The following table presents the fair value of White Mountains’s mortgage and asset-backed securities and collateralized loan obligations as of December 31, 2025 and 2024:

	December 31, 2025						December 31, 2024
Millions	Fair Value		Level 2		Level 3		Fair Value		Level 2		Level 3
Mortgage-backed securities:
Agency:
FNMA	$	192.1	$	192.1	$	—	$	198.3	$	198.3	$	—
FHLMC	141.3		141.3		—		147.1		147.1		—
GNMA	20.6		20.6		—		24.2		24.2		—
Total agency (1)	354.0		354.0		—		369.6		369.6		—
Non-agency: Commercial	—		—		—		.4		.4		—
Total non-agency	—		—		—		.4		.4		—
Total mortgage-backed securities	354.0		354.0		—		370.0		370.0		—
Other asset-backed securities:
Vehicle receivables	21.9		21.9		—		1.7		1.7		—
Credit card receivables	14.0		14.0		—		.2		.2		—
Other	3.1		3.1		—		2.0		2.0		—
Total other asset-backed securities	39.0		39.0		—		3.9		3.9		—
Total mortgage and asset-backed securities	393.0		393.0		—		373.9		373.9		—
Collateralized loan obligations	347.0		347.0		—		236.7		236.7		—
Total mortgage and asset-backed securities and    collateralized loan obligations	$	740.0	$	740.0	$	—	$	610.6	$	610.6	$	—

(1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guarantee of the U.S. Government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).

Schedule of carrying values for other long-term investments

The following tables present the carrying values of White Mountains’s other long-term investments by reportable segment as of December 31, 2025 and 2024:

	Fair Value as of December 31, 2025
Millions	Ark/WM Outrigger		Kudu		Other		Total
Kudu’s Participation Contracts	$	—	$	1,285.0	$	—	$	1,285.0
Bamboo SPV	—		—		250.0		250.0
PassportCard/DavidShield	—		—		170.0		170.0
BroadStreet SPV	—		—		160.0		160.0
Elementum	—		—		35.0		35.0
Other unconsolidated entities (1)	—		—		73.7		73.7
Total unconsolidated entities	—		1,285.0		688.7		1,973.7
Private equity funds and hedge funds	167.1		—		228.1		395.2
Bank loan fund	308.5		—		—		308.5
Lloyd’s trust deposits	180.4		—		—		180.4
ILS funds	—		—		50.1		50.1
Private debt instruments	—		6.4		10.5		16.9
Other	33.7		—		—		33.7
Total other long-term investments	$	689.7	$	1,291.4	$	977.4	$	2,958.5

(1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, private equity securities and limited liability company units.

	Fair Value as of December 31, 2024
Millions	Ark/WM Outrigger		Kudu		Other		Total
Kudu’s Participation Contracts	$	—	$	1,008.4	$	—	$	1,008.4
PassportCard/DavidShield	—		—		150.0		150.0
Elementum	—		—		35.0		35.0
Other unconsolidated entities (1)	—		—		63.6		63.6
Total unconsolidated entities	—		1,008.4		248.6		1,257.0
Private equity funds and hedge funds	104.1		—		256.5		360.6
Bank loan fund	264.7		—		—		264.7
Lloyd’s trust deposits	149.9		—		—		149.9
ILS funds	—		—		74.0		74.0
Private debt instruments	—		5.6		9.3		14.9
Other	29.1		—		—		29.1
Total other long-term investments	$	547.8	$	1,014.0	$	588.4	$	2,150.2

(1) Includes White Mountains’s noncontrolling equity interests in certain preferred securities, limited liability company units, limited partnership units and Simple Agreement for Future Equity (“SAFE”) investments.

Schedule of investments and unfunded commitments by investment objective and sector

The following table presents the fair value of investments and unfunded commitments in private equity funds and hedge funds by investment objective and sector as of December 31, 2025 and 2024:

	December 31, 2025				December 31, 2024
Millions	Fair Value		Unfunded Commitments		Fair Value		Unfunded Commitments
Private equity funds
Aerospace/Defense/Government	$	144.0	$	39.9	$	152.4	$	49.3
Financial services	104.2		23.7		93.4		32.0
Real estate	3.0		2.2		3.7		2.4
Total private equity funds	251.2		65.8		249.5		83.7
Hedge funds
Long/short all cap global	110.6		—		51.9		—
Long/short equity financials and business services	33.4		—		59.2		—
Total hedge funds	144.0		—		111.1		—
Total private equity funds and hedge funds	$	395.2	$	65.8	$	360.6	$	83.7

Schedule investments in private equity funds subject to lock-up periods

The following table presents investments in private equity funds that were subject to lock-up periods as of December 31, 2025:

Millions	1 – 3 years		3 – 5 years		5 – 10 years		>10 years		Total
Private equity funds — expected lock-up period remaining	$	57.2	$	8.0	$	161.7	$	24.3	$	251.2

Schedule of fair value investments by level

The following table presents the changes in White Mountains’s fair value measurements for Level 3 investments for the years ended December 31, 2025 and 2024:

Level 3 Investments
Other Long-term Investments
Millions	December 31, 2025		December 31, 2024
Beginning balance	$	1,262.7	$	1,138.2
Net realized and unrealized gains	141.0		54.4
Purchases and contributions	202.2		111.5
Effect of Bamboo Sale Transaction (1)	250.0		—
Effect of Distinguished Transaction (1)	(7.1)		—
Sales and distributions	(34.4)		(41.4)
Transfers in	—		—
Transfers out	—		—
Ending balance	$	1,814.4	$	1,262.7

(1) See Note 2 — “Significant Transactions.”

Schedule of significant unobservable inputs used in estimating the fair value of other long-term investments

The following tables present significant unobservable inputs used in estimating the fair value of White Mountains’s other long-term investments, classified within Level 3 as of December 31, 2025 and 2024. The tables below exclude $250.4 million and $23.4 million of Level 3 other long-term investments generally valued based on recent or expected transaction prices. As of December 31, 2025, the Bamboo SPV was measured at fair value based on the value implied in the Bamboo Sale Transaction. See Note 2 — “Significant Transactions.” The fair value of investments in the BroadStreet SPV, private equity funds and hedge funds, bank loan funds, Lloyd’s trust deposits and ILS funds are generally estimated using NAV. Investments for which fair value is measured at NAV are not classified within the fair value hierarchy.

$ in Millions	December 31, 2025
Description	Valuation Technique(s) (1)	Fair Value (2)	Unobservable Inputs
			Discount Rate (5)	Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (5)
Kudu’s Participation Contracts (3) (4)	Discounted cash flow	$1,285.0	16% - 25%	7x - 22x
PassportCard/DavidShield	Discounted cash flow	$170.0	24%	4%
Elementum	Discounted cash flow	$35.0	22%	3%
Preferred securities	Discounted cash flow	$35.6	8%	N/A
Private equity securities	Discounted cash flow	$21.9	35%	4%
Private debt instruments	Discounted cash flow	$16.5	11% - 12%	N/A

(1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates.

(2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

(3) Since Kudu’s Participation Contracts are not subject to corporate taxes within Kudu Investment Management, LLC, pre-tax discount rates are applied to pre-tax cash flows in determining fair values. The weighted average discount rate and weighted average terminal cash flow exit multiple applied to Kudu’s Participation Contracts was 19% and 14x.

(4) In 2025, Kudu contributed $201.7 into new and existing Participation Contracts.

(5) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements.

$ in Millions	December 31, 2024
Description	Valuation Technique(s) (1)	Fair Value (2)	Unobservable Inputs
			Discount Rate (5)	Terminal Cash Flow Exit Multiple (x) or Terminal Revenue Growth Rate (%) (5)
Kudu’s Participation Contracts (3) (4)	Discounted cash flow	$1,008.4	17% - 25%	7x - 22x
PassportCard/DavidShield	Discounted cash flow	$150.0	24%	4%
Elementum	Discounted cash flow	$35.0	22%	4%
Preferred securities	Discounted cash flow	$31.4	9%	N/A
Private debt instruments	Discounted cash flow	$14.5	11% - 12%	N/A

(1) Key inputs to the discounted cash flow analysis generally include projections of future revenue and earnings, discount rates and terminal exit multiples or growth rates.

(2) Includes the net unrealized investment gains (losses) associated with foreign currency; foreign currency effects based on observable inputs.

(3) Since Kudu’s Participation Contracts are not subject to corporate taxes within Kudu Investment Management, LLC, pre-tax discount rates are applied to pre-tax cash flows in determining fair values. The weighted average discount rate and weighted average terminal cash flow exit multiple applied to Kudu’s Participation Contracts was 19% and 14x.

(4) In 2024, Kudu contributed $103.5 into new and existing Participation Contracts.

(5) Increases (decreases) to the discount rates in isolation would result in lower (higher) fair value measurements, while increases (decreases) to the terminal cash flow exit multiples or terminal revenue growth rates in isolation would result in higher (lower) fair value measurements.