Municipal Bond Guarantee Insurance (Details) $ in Millions	1 Months Ended	12 Months Ended
	Apr. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) Contract	Dec. 31, 2018 USD ($) Contract	Dec. 31, 2017 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2020	Sep. 30, 2019 USD ($)	Jul. 31, 2012 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured		15.00%
Percentage of risk premium ceded			60.00%
Assets Held-in-trust		$ 786.7	$ 757.4
Interest receivable		$ 162.7	$ 143.7
HG Re
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured			15.00%
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]
Surplus notes					503.0
Percent over a target trust balance		102.00%
Cash and fixed income securities								$ 100.0
Supplement Trust target		$ 603.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Contracts outstanding | Contract		8,987	7,525
Remaining weighted average contract period (in years)		10 years 8 months 12 days
Municipal bond outstanding principal		$ 62,250.5	$ 52,201.6				$ 1,100.0
Municipal bond outstanding interest		31,799.7	26,560.3
Municipal bond debt service outstanding		94,050.2	78,761.9
Unearned premium revenue		198.4	$ 176.0
January 1, 2020 - March 31, 2020		4.8
April 1, 2020 - June 30, 2020		4.8
July 1, 2020 - September 30, 2020		4.6
October 1, 2020 - December 31, 2020		4.5
2020		18.7
2021		17.3
2022		16.3
2023		15.2
2024		14.2
2025 and thereafter		$ 116.7
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%
Common Stock | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)		88.40%	88.40%
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes		$ 457.6	$ 481.3
Accrued interest on surplus notes		162.7	143.7
Supplemental Trust [Member] | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Special cash payment		23.7	17.7	$ 4.0
Accrued interest on surplus notes		0.3	0.7	0.1
Surplus Note
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Assets Held-in-trust		457.6	481.3
Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Special cash payment		32.0	23.0	5.0
Accrued interest on surplus notes		$ 8.0	$ 4.6	0.9
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	12 years
Collateral Financial Guarantee, Callable Term		5 years
Liabilities for guarantees on long-duration contracts, guaranteed benefit liability, gross	$ 165.0
Financial guarantee insurance contracts, reimbusement	$ 100.0
Percentage of Quota Share Reinsurance Agreement			100.00%
Direct written premiums			$ 2,200.0
Maximum | Fidus Re Ltd. | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Liabilities for guarantees on long-duration contracts, guaranteed benefit liability, gross		$ 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
Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Special cash payment		32.0	23.0
Accrued interest on surplus notes		$ 8.0	$ 4.6
Fixed interest rate | Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)		8.00%
Forecast | Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)						4.60%