Statutory Capital and Surplus (Details) - USD ($)	1 Months Ended			12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Jan. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2012
Statutory Accounting Practices [Line Items]
Cash and investments included in collateral trust	$ 481,700,000			$ 481,700,000
Other long-term investments	1,377,800,000	$ 786,800,000		1,377,800,000	$ 786,800,000
Total assets	7,000,700,000	4,831,400,000		7,000,700,000	4,831,400,000
Lloyds Bank
Statutory Accounting Practices [Line Items]
Total assets	343,600,000			343,600,000
HG Global-BAM
Statutory Accounting Practices [Line Items]
Total assets	1,044,800,000	1,017,800,000		1,044,800,000	1,017,800,000
BAM
Statutory Accounting Practices [Line Items]
Surplus notes	364,600,000	388,200,000		364,600,000	388,200,000
Interest receivable	5,700,000	4,400,000		5,700,000	4,400,000
Accrued interest on surplus notes	157,600,000	155,700,000		157,600,000	155,700,000
Ark
Statutory Accounting Practices [Line Items]
Total assets	3,027,000,000	0		3,027,000,000	0
NSM
Statutory Accounting Practices [Line Items]
Distributions made				8,000,000
Kudu
Statutory Accounting Practices [Line Items]
Distributions made				19,400,000
Total assets	727,100,000	430,200,000		727,100,000	430,200,000
Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash	453,500,000			453,500,000
Other long-term investments	170,500,000			170,500,000
Total assets	1,208,400,000	2,381,500,000		1,208,400,000	2,381,500,000
HG Global
Statutory Accounting Practices [Line Items]
Collateral held in supplement trust	852,000,000	827,100,000		852,000,000	827,100,000
Cash and investments included in collateral trust	481,700,000	434,500,000		481,700,000	434,500,000
Interest receivable	157,600,000	155,700,000		157,600,000	155,700,000
HG Re | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	760,000,000			760,000,000
Surplus required	1			1
Cash outside of collateral trusts	3,100,000			3,100,000
Collateral held in supplement trust	852,000,000			852,000,000
Cash and investments outside of collateral trusts	9,400,000			9,400,000
Accrued interest held in supplemental trust	116,800,000			116,800,000
BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	298,100,000			298,100,000
Surplus required	66,000,000			66,000,000
Statutory net loss				(49,300,000)	(59,300,000)	$ (38,300,000)
Ark
Statutory Accounting Practices [Line Items]
Other long-term investments	326,200,000	0		326,200,000	0
Total assets	3,027,000,000	0		3,027,000,000	0
Ark | Ark
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	780,300,000			780,300,000
Surplus required	$ 288,200,000			$ 288,200,000
Percent of statutory capital balance	15.00%			15.00%
Percent of statutory capital and surplus balance	25.00%			25.00%
Dividend or capital distributions, maximum				$ 113,600,000
Statutory surplus balance	$ 757,600,000			757,600,000
Unrestricted cash	4,000,000			$ 4,000,000
HG Global
Statutory Accounting Practices [Line Items]
Percent over a target trust balance				102.00%
Supplement trust target	603,000,000			$ 603,000,000
Surplus notes							$ 503,000,000
HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Preferred stock value	$ 619,000,000			$ 619,000,000
Ownership interest (as a percent)	96.90%			96.90%
Preferred stock dividend rate (as a percent)				6.00%
NSM | NSM
Statutory Accounting Practices [Line Items]
Unrestricted cash	$ 22,300,000			$ 22,300,000
Kudu
Statutory Accounting Practices [Line Items]
Unrestricted cash	16,900,000			16,900,000
Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Cash repayments on notes	33,800,000	30,100,000	$ 65,000,000
Accrued interest on surplus notes	$ 9,800,000	8,400,000	16,200,000	$ 9,800,000	8,400,000
Surplus Note | BAM | U.S. treasury rate plus
Statutory Accounting Practices [Line Items]
Basis spread on variable rate				3.00%
Effective yield (as a percent)	3.20%			3.20%
Surplus Note | BAM | Fixed interest rate
Statutory Accounting Practices [Line Items]
Interest rate	8.00%			8.00%
Supplemental Trust | BAM
Statutory Accounting Practices [Line Items]
Cash repayments on notes	$ 23,600,000	21,500,000	47,900,000
Accrued interest on surplus notes	400,000	$ 200,000	$ 900,000	$ 400,000	$ 200,000
Preferred stocks | HG Global
Statutory Accounting Practices [Line Items]
Dividends				22,000,000
Preferred stocks | HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends payable	414,700,000			414,700,000
Preferred stocks | White Mountains | HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends				21,300,000
Dividends payable	400,500,000			400,500,000
Common Equities | Other Operations
Statutory Accounting Practices [Line Items]
Distributions made				3,100,000
Common Equities | MediaAlpha
Statutory Accounting Practices [Line Items]
Common stock	$ 261,600,000			$ 261,600,000