Municipal Bond Guarantee Insurance (Details) $ in Millions	1 Months Ended	12 Months Ended
	Apr. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Contract	Dec. 31, 2017 USD ($) yr Contract	Dec. 31, 2016 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2019
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Supplement Trust Target		$ 603.0
Percentage of par value of policy reinsured		15.00%	15.00%
Percentage of risk premium ceded			60.00%
Repayments of debt, principal		$ 15.4	$ 365.0	$ 404.6
Interest paid		8.8	$ 1.4	$ 2.1
Percentage of discounted future cash flows			10.00%
Unearned premium revenue		176.0	$ 136.8
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
Assets Held-in-trust		$ 100.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Year of U.S. Treasury Rate | yr			1
Contracts outstanding | Contract		7,525	6,371
Remaining weighted average contract period (in years)		10 years 8 months	10 years 11 months
Municipal bond outstanding principal		$ 52,201.6	$ 42,090.6
Municipal bond outstanding interest		26,560.3	21,057.1
Municipal bond debt service outstanding		78,761.9	$ 63,147.7
Unearned premium revenue		176.0
January 1, 2019 - March 31, 2019		4.1
April 1, 2019 - June 30, 2019		4.0
July 1, 2019 - September 30, 2019		4.0
October 1, 2019 - December 31, 2019		3.8
2019		15.9
2020		15.0
2021		14.0
2022		13.2
2023		12.3
2024 and thereafter		$ 105.6
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%
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured				15.00%
Collateral held in supplement trust		$ 757.4	$ 715.1
Interest receivable		143.7	126.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes		$ 481.3	$ 499.0
Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest rate (as a percent)		4.60%	3.78%
Basis spread on variable rate		300.00%
Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Repayments of debt, principal		$ 17.7	$ 4.0
Interest paid		5.3	$ 1.0
Combination One | Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of years of change in interest rate | yr			5
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)						5.70%
Series A BAM Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes			$ 203.0
Series B BAM Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes		$ 300.0
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	12 years
Collateral Financial Guarantee, Callable Term		5 years
Liabilities for guarantees on long-duration contracts, guaranteed benefit liability, gross	$ 165.0
Percentage of Quota Share Reinsurance Agreement		100.00%
Financial guarantee insurance contracts, reimbusement	100.0
Direct written premiums		$ 2,200.0
Minimum | 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	$ 165.0
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		$ 276.0