Statutory Capital and Surplus (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statutory Accounting Practices [Line Items]
Repayments of debt, principal			$ 22,900,000	$ 15,400,000	$ 365,000,000.0
Other long-term investments		$ 856,000,000	856,000,000
Kudu
Statutory Accounting Practices [Line Items]
Proceeds from borrowings		57,000,000.0
Distributions made		53,700,000
BAM
Statutory Accounting Practices [Line Items]
Accrued interest on surplus notes		162,700,000	162,700,000	143,700,000
Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash		602,600,000	602,600,000
Other long-term investments		202,300,000	202,300,000
HG Re | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		745,400,000	745,400,000
Surplus required		1	1
Cash outside of collateral trusts		3,000,000.0	3,000,000.0
Collateral held in supplement trust		786,700,000	786,700,000
Cash and investments outside of collateral trusts		6,300,000	6,300,000
Accrued interest held in Supplemental Trust		111,200,000	111,200,000
BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		402,400,000	402,400,000
Statutory net income (loss)			(38,300,000)	(34,600,000)	(25,400,000)
Surplus required		66,000,000.0	$ 66,000,000.0
HG Global
Statutory Accounting Practices [Line Items]
Percent over a target trust balance			102.00%
Supplement Trust target		603,000,000.0	$ 603,000,000.0
HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Preferred stock, value		$ 619,000,000.0	$ 619,000,000.0
Ownership interest (as a percent)		96.90%	96.90%
Preferred stock dividend rate			6.00%
White Mountains | Kudu
Statutory Accounting Practices [Line Items]
Distributions made		$ 53,300,000
NSM Insurance HoldCo, LLC | NSM Insurance HoldCo, LLC
Statutory Accounting Practices [Line Items]
Cash		33,400,000	$ 33,400,000
Kudu
Statutory Accounting Practices [Line Items]
Cash and short-term investments		5,900,000	5,900,000
Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Special cash payment			32,000,000.0	23,000,000.0	5,000,000.0
Accrued interest on surplus notes		8,000,000.0	8,000,000.0	4,600,000	$ 900,000
Preferred stocks | HG Global
Statutory Accounting Practices [Line Items]
Dividends payable		2,000,000.0	2,000,000.0
Preferred stocks | HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends payable		341,700,000	341,700,000
Preferred stocks | White Mountains | HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends payable		330,200,000	330,200,000
Common Equities | Other Operations
Statutory Accounting Practices [Line Items]
Distributions made				3,200,000
Common equity securities		683,900,000	683,900,000
Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Accrued interest held in Supplemental Trust		300,000	300,000	700,000
Special cash payment			32,000,000.0	23,000,000.0
Accrued interest on surplus notes		$ 8,000,000.0	8,000,000.0	4,600,000
Repayments of debt, principal			$ 23,700,000	$ 17,700,000
Subsequent Event | BAM
Statutory Accounting Practices [Line Items]
Accrued interest on surplus notes	$ 146,500,000
Subsequent Event | Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Accrued interest held in Supplemental Trust	900,000
Special cash payment	65,000,000.0
Accrued interest on surplus notes	16,200,000
Repayments of debt, principal	$ 47,900,000