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Municipal Bond Guarantee - Narrative (Details) $ in Millions	1 Months Ended		3 Months Ended				12 Months Ended
	Jan. 31, 2020 USD ($)	Apr. 30, 2018 USD ($)	Mar. 31, 2020 USD ($) Contract	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 yr	Dec. 31, 2019 USD ($) Contract	Dec. 31, 2012 USD ($)	Dec. 31, 2024	Dec. 31, 2020	Sep. 30, 2019 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items]
Ownership interest (as a percent)			50.00%				50.00%
Repayments of debt			$ 1.0	$ 14.8
Interest paid			$ 5.2	$ 3.7
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution								$ 594.5
Surplus notes													$ 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 Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)			88.40%				88.40%
BAM
Guarantor Obligations [Line Items]
Percentage of risk premium ceded			60.00%
Year of U.S. Treasury rate \| yr						1
Contracts outstanding \| Contract			9,312				8,987
Remaining weighted average contract period outstanding (in years)			10 years 7 months 6 days				10 years 8 months 12 days
Municipal bond outstanding principal			$ 64,481.2				$ 62,250.5				$ 1,100.0
Municipal bond outstanding interest			32,449.1				31,799.7
Municipal bond debt service outstanding			96,930.3				94,050.2
April 1, 2020 - December 31, 2020			14.3
2021			18.0
2022			16.9
2023			15.9
2024			14.8
2025			13.8
2026 and thereafter			109.0
Total gross unearned insurance premiums			202.7				198.4
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust			792.7				786.7
Interest receivable			$ 150.4				162.7
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured			15.00%
BAM
Guarantor Obligations [Line Items]
Surplus notes			$ 409.7				$ 457.6
BAM Management
Guarantor Obligations [Line Items]
Partners capital contribution								$ 14.5
Supplemental Trust [Member] \| BAM
Guarantor Obligations [Line Items]
Repayments of Unsecured Debt			47.9
Accrued Interest Payment on Surplus Notes			$ 0.9
Surplus Note \| BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)			10.00%
Repayments of Unsecured Debt			$ 65.0
Accrued Interest Payment on Surplus Notes			$ 16.2
Combination One \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate \| yr						5
US Treasury Rate \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Basis spread on variable rate			3.00%
Fixed interest rate \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)					8.00%				8.00%
First Quarter, year two \| BAM
Guarantor Obligations [Line Items]
2021			$ 4.6
Second Quarter, year two \| BAM
Guarantor Obligations [Line Items]
2021			4.5
Third Quarter, year two \| BAM
Guarantor Obligations [Line Items]
2021			4.5
Fourth Quarter, year two \| BAM
Guarantor Obligations [Line Items]
2021			$ 4.4
Series A BAM Surplus Note \| HG Global
Guarantor Obligations [Line Items]
Surplus notes												$ 203.0	203.0
Series B BAM Surplus Note \| HG Global
Guarantor Obligations [Line Items]
Surplus notes													$ 300.0
BAM \| Fidus Re Ltd.
Guarantor Obligations [Line Items]
Losses above retention, percentage of liability		90.00%
BAM \| HG Global
Guarantor Obligations [Line Items]
Term of Agreement	10 years
Fidus Re Ltd. \| BAM
Guarantor Obligations [Line Items]
Secured Debt		$ 100.0
Collateralized financial guarantee, term		12 years
Collateral financial guarantee, callable term			5 years
Aggregate losses on ceded business		$ 165.0
Premium receivable, Premium expected to be collected		100.0
Percentage of Quota Share Reinsurance Agreement					100.00%
Direct written premiums					$ 2,200.0
Maximum \| BAM \| HG Global
Guarantor Obligations [Line Items]
Aggregate losses on ceded business	$ 75.0
Maximum \| Fidus Re Ltd. \| BAM
Guarantor Obligations [Line Items]
Aggregate losses on ceded business		$ 276.1
Scenario, Forecast \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)							5.70%			4.60%