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Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($) $ in Millions		1 Months Ended					3 Months Ended	12 Months Ended
	Feb. 17, 2021	Jan. 31, 2021	Dec. 31, 2020	Jan. 31, 2020	Nov. 30, 2018	Apr. 30, 2018	Mar. 31, 2021	Dec. 31, 2012	Dec. 31, 2021	Sep. 30, 2019	Dec. 31, 2017	Jul. 31, 2012
Guarantor Obligations [Line Items]
Percentage of risk premium ceded							60.00%
Insurance-linked securities, term						12 years
Insurance-linked securities, callable term						5 years
Percent change in discounted future cash flows							10.00%
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust			$ 827.1				$ 819.1
Collateral Trusts, Investments And Cash			434.5				427.9
Interest receivable			155.7				158.8
BAM
Guarantor Obligations [Line Items]
Surplus notes			388.2				388.2
Interest receivable			$ 4.4				$ 3.0
HG Global
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured							15.00%
Capital invested								$ 594.5
Surplus notes								503.0				$ 503.0
Cash and Fixed Income Securities												100.0
Percent over a target trust balance							102.00%
Supplement Trust Target							$ 603.0
HG Global \| Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)			96.90%				96.90%
HG Global \| Common equity securities
Guarantor Obligations [Line Items]
Ownership interest (as a percent)			88.40%				88.40%
HG Global \| Series B BAM Surplus Note
Guarantor Obligations [Line Items]
Surplus notes												300.0
HG Global \| Series A BAM Surplus Note
Guarantor Obligations [Line Items]
Surplus notes											$ 203.0	$ 203.0
Restricted Cash			$ 604.3				$ 597.9
HG Global \| BAM \| Maximum
Guarantor Obligations [Line Items]
Aggregate losses on ceded business				$ 75.0
HG Global \| Non-controlling interest
Guarantor Obligations [Line Items]
Capital invested								$ 14.5
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured							15.00%
Restricted Cash			222.8				$ 221.2
BAM
Guarantor Obligations [Line Items]
Principal		$ 800.0	75,287.7				$ 77,689.3			$ 1,100.0
Additional municipal bond guarantee										$ 36.9
BAM \| Surplus Note
Guarantor Obligations [Line Items]
Repayments of unsecured debt			30.1	65.0
Accrued interest payment on surplus notes			8.4	16.2
BAM \| Supplemental Trust
Guarantor Obligations [Line Items]
Repayments of unsecured debt			21.5	47.9
Accrued interest payment on surplus notes			$ 0.2	$ 0.9
BAM \| US Treasury (UST) Interest Rate \| Surplus Note
Guarantor Obligations [Line Items]
Basis spread on variable rate							3.00%
BAM \| Fixed interest rate \| Surplus Note
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)							8.00%
BAM \| Forecast \| US Treasury (UST) Interest Rate \| Surplus Note
Guarantor Obligations [Line Items]
Effective percentage									3.10%
BAM \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Secured Debt	$ 150.0					$ 100.0
Aggregate losses on ceded business	135.0					165.0
Premium receivable, Premium expected to be collected	150.0					100.0
Percentage Of Financial Guaranty Policies Covered							41.00%
Principal					$ 2,200.0
Percentage of quota share reinsurance agreement		100.00%			100.00%
BAM \| Fidus Re Ltd. \| Maximum
Guarantor Obligations [Line Items]
Aggregate losses on ceded business	$ 301.7					$ 276.1
Fidus Re Ltd. \| BAM
Guarantor Obligations [Line Items]
Losses above retention, percentage of liability	90.00%					90.00%