Municipal Bond Guarantee - Narrative (Details) $ in Millions	1 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) Contract	Apr. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) yr Contract	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Contract	Dec. 31, 2017 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2019	Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	50.00%		50.00%		50.00%
Repayments of debt			$ 22.2	$ 11.2
Interest paid			11.7	$ 5.3
Percentage of discounted future cash flows						10.00%
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution							$ 594.5
Surplus notes	$ 481.3		$ 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%		96.90%
HG Global | Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	88.40%		88.40%		88.40%
BAM
Guarantor Obligations [Line Items]
Percentage of risk premium ceded	60.00%		60.00%
Year of U.S. Treasury rate | yr			1
Contracts outstanding | Contract	8,508		8,508		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	$ 59,190.9		$ 59,190.9		$ 52,201.6
Municipal bond outstanding interest	30,701.7		30,701.7		26,560.3
Municipal bond debt service outstanding	89,892.6		89,892.6		78,761.9
October 1, 2019 - December 31, 2019	4.9		4.9
2020	18.7		18.7
2021	17.4		17.4
2022	16.3		16.3
2023	15.3		15.3
2024	14.3		14.3
2025 and thereafter	116.6		116.6
Total gross unearned insurance premiums	203.5		203.5		176.0
HG Global
Guarantor Obligations [Line Items]
Collateral held in supplement trust	787.4		787.4		757.4
Interest receivable	164.2		$ 164.2		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		$ 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%		8.00%
First Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020	$ 4.8		$ 4.8
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020	4.8		4.8
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020	4.6		4.6
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2020	4.5		$ 4.5
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	$ 1,100.0				$ 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%