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Municipal Bond Guarantee Insurance (Details) - USD ($) $ in Millions		1 Months Ended				12 Months Ended
	Nov. 30, 2018	Dec. 31, 2020	Jan. 31, 2020	Dec. 31, 2019	Apr. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2012	Dec. 31, 2021	Jun. 30, 2020	Sep. 30, 2019	Dec. 31, 2017	Jul. 31, 2012
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured		15.00%				15.00%
Percentage of risk premium ceded		60.00%				60.00%
Cash and investments included in collateral trust		$ 434.5				$ 434.5
Percent change in discounted future cash flows						10.00%
HG Re
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured		15.00%				15.00%
Trust balance		$ 222.8		$ 190.3		$ 222.8	$ 190.3
White Mountains
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions								$ 594.5
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured		15.00%				15.00%
Surplus notes								503.0
Cash and fixed income securities													$ 100.0
Percent over a target trust balance						102.00%
Supplement trust target		$ 603.0				$ 603.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Principal		$ 75,287.7		$ 62,250.5		$ 75,287.7	$ 62,250.5			$ 36.9	$ 1,100.0
BAM Management
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions								$ 14.5
Preferred stocks \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)		96.90%		96.90%		96.90%	96.90%
Common equity securities \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)		88.40%		88.40%		88.40%	88.40%
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Collateral held in supplement trust		$ 827.1		$ 786.7		$ 827.1	$ 786.7
Cash and investments included in collateral trust		434.5		321.6		434.5	321.6
Interest receivable		155.7		162.7		155.7	162.7
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes		388.2		457.6		388.2	457.6
Interest receivable		4.4		7.5		4.4	7.5
Accrued interest on surplus notes		155.7		162.7		155.7	162.7
Supplemental Trust \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Cash repayments on notes		21.5	$ 47.9	23.7
Accrued interest on surplus notes		0.2	0.9	0.3		0.2	0.3
Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Cash repayments on notes		30.1	65.0	32.0			10.0
Accrued interest on surplus notes		8.4	16.2	8.0		$ 8.4	8.0
One-time cash payment				10.0
BAM \| Fidus Re Ltd.
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Losses above retention, percentage of liability					90.00%
Fidus Re Ltd. \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Insurance linked securities					$ 100.0
Collateralized financial guarantee term (in years)					12 years
Collateral financial guarantee, callable term (in years)						5 years
Aggregate loss limit on contracts					$ 165.0
Reimbursement guarantee under contracts					$ 100.0
Covered Portfolio rate						42.00%
Percentage of quota share reinsurance agreement	100.00%
Direct written premiums	$ 2,200.0
Maximum \| BAM \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Aggregate loss limit on contracts			$ 75.0
Maximum \| Fidus Re Ltd. \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Aggregate loss limit on contracts		276.1				$ 276.1
Series B BAM Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes													$ 300.0
Series A BAM Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes												$ 203.0
Trust balance		604.3		596.4		604.3	596.4
Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Cash repayments on notes						30.1	32.0
Accrued interest on surplus notes		$ 8.4		$ 8.0		$ 8.4	$ 8.0
Fixed interest rate \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate		8.00%				8.00%
U.S. treasury rate plus \| Surplus Note \| BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Basis spread on variable rate						3.00%
U.S. treasury rate plus \| Surplus Note \| BAM \| Forecast
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Effective yield (as a percent)									3.10%