Municipal Bond Guarantee - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2018 USD ($)	Mar. 31, 2019 USD ($) yr Contract	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) Contract	Dec. 31, 2017 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2019	Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items]
Repayments of debt		$ 14.8	$ 2.3
Interest paid		3.7	$ 0.3
Percentage of discounted future cash flows					10.00%
Total gross unearned insurance premiums		179.6		$ 176.0
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution						$ 594.5
Surplus notes		$ 481.3						$ 503.0
Assets held-in-trust				$ 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		7,786		7,525
Remaining weighted average contract period outstanding (in years)		10 years 8 months 12 days		10 years 8 months 12 days
Municipal bond outstanding principal		$ 53,715.2		$ 52,201.6
Municipal bond outstanding interest		27,348.9		26,560.3
Municipal bond debt service outstanding		81,064.1		78,761.9
April 1, 2019 - December 31, 2019		12.2
2020		15.5
2021		14.5
2022		13.6
2023		12.8
2024		12.0
2025 and thereafter		99.0
Total gross unearned insurance premiums		179.6
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust		771.5		757.4
Interest receivable		$ 150.5		143.7
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured		15.00%
BAM
Guarantor Obligations [Line Items]
Surplus notes		$ 481.3		$ 481.3
BAM Management
Guarantor Obligations [Line Items]
Partners capital contribution						$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, fixed interest rate (as a percent)				4.60%
Surplus Note | BAM
Guarantor Obligations [Line Items]
Repayments of debt				$ 17.7
Interest paid				$ 5.3
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]
2020		$ 3.9
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020		3.9
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020		3.9
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020		3.8
Series A BAM Surplus Note | HG Global
Guarantor Obligations [Line Items]
Surplus notes					$ 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%
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
Minimum | Fidus Re Ltd. | BAM
Guarantor Obligations [Line Items]
Aggregate losses on ceded business	$ 165.0
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%