Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2022	Dec. 31, 2021	Jan. 31, 2021	Mar. 31, 2021	Jun. 30, 2018	Sep. 30, 2022	Dec. 31, 2022	Sep. 01, 2022	Jan. 31, 2020	Jul. 31, 2012
Guarantor Obligations [Line Items]
Percentage of risk premium ceded	60.00%					60.00%
Insurance-linked securities, term				12 years	12 years
Insurance-linked securities, callable term				5 years	5 years
Debt	$ 615.5	$ 420.9				$ 615.5
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust	833.9	852.0				833.9
Collateral trusts, investments and cash	457.7	481.7				457.7
BAM
Guarantor Obligations [Line Items]
Surplus notes	364.6	364.6				364.6
Interest receivable	11.6	5.7				11.6
HG Global
Guarantor Obligations [Line Items]
Surplus notes	$ 503.0					$ 503.0				$ 503.0
Percentage of par value of policy reinsured	15.00%					15.00%
Percent over a target trust balance						102.00%
Supplement trust target	$ 603.0					$ 603.0
HG Global | Series A BAM Surplus Note
Guarantor Obligations [Line Items]
Assets Held-in-trust	$ 574.5	601.8				$ 574.5
HG Global | BAM | Maximum
Guarantor Obligations [Line Items]
Aggregate losses on ceded business									$ 125.0
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured	15.00%					15.00%
Assets Held-in-trust	$ 259.4	250.2				$ 259.4
BAM
Guarantor Obligations [Line Items]
Principal	98,619.0	89,196.5	$ 805.5			98,619.0		$ 42.5
BAM | HG Global
Guarantor Obligations [Line Items]
Interest receivable	166.4	157.6				166.4
BAM | Surplus Note
Guarantor Obligations [Line Items]
Repayments of unsecured debt		33.8
Accrued interest payment on surplus notes		9.8
BAM | Surplus Note | HG Global
Guarantor Obligations [Line Items]
Debt	$ 364.6	364.6				$ 364.6
BAM | Supplemental Trust
Guarantor Obligations [Line Items]
Repayments of unsecured debt		23.6
Accrued interest payment on surplus notes		$ 0.4
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%					8.00%
BAM | Forecast | US Treasury (UST) Interest Rate | Surplus Note
Guarantor Obligations [Line Items]
Effective percentage							3.20%
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 quota share reinsurance agreement	100.00%		100.00%
BAM | Fidus Re Ltd. | 2018 Covered Portfolio
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered						28.00%
BAM | Fidus Re Ltd. | 2021 Covered Portfolio
Guarantor Obligations [Line Items]
Percentage of financial guaranty policies covered						33.00%
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%