Municipal Bond Guarantee (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) yr Contract	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Contract	Dec. 31, 2012 USD ($)	Jun. 30, 2017 USD ($)	Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items]
Pre-tax income (loss)	$ 8.4	$ (13.8)	$ 2.6	$ (72.8)
Net investment income (loss) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
HG Global
Guarantor Obligations [Line Items]
Partners capital contribution						$ 594.5
Surplus notes								$ 503.0
Income (loss) from continuing operations before income taxes, domestic	$ 7.0		$ 20.3
HG Global | Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)	96.90%		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%		88.40%
BAM
Guarantor Obligations [Line Items]
Surplus notes	$ 500.0		$ 500.0
Year of U.S. Treasury rate | yr			1
Municipal bond contract | Contract	5,907		5,907		4,807
Remaining weighted average contract period outstanding (in years)			10 years 11 months		10 years 10 months
Municipal bond outstanding principal	$ 39,207.5		$ 39,207.5		$ 33,057.3
Municipal bond outstanding interest	19,681.0		19,681.0		16,396.6
Municipal bond debt service outstanding	58,888.5		58,888.5		49,453.9
October 1, 2017 - December 31, 2017	2.5		2.5
2018	9.9		9.9
2019	9.5		9.5
2020	9.2		9.2
2021	8.7		8.7
2022 and thereafter	78.7		78.7
Total gross unearned insurance premiums	118.5		118.5		$ 82.9
Build America Mutual
Guarantor Obligations [Line Items]
Income (loss) from continuing operations before income taxes, domestic	(11.9)		(35.6)
HG Global
Guarantor Obligations [Line Items]
Pre-tax income (loss)	7.0	5.2	20.3	18.4
Net investment income (loss) - BAM Surplus Note interest	4.8	4.5	$ 14.3	13.4
HG Re
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured			15.00%
BAM
Guarantor Obligations [Line Items]
Pre-tax income (loss)	(11.9)	(13.7)	$ (35.6)	(30.3)
Net investment income (loss) - BAM Surplus Note interest	(4.8)	$ (4.5)	(14.3)	$ (13.4)
BAM Management
Guarantor Obligations [Line Items]
Partners capital contribution						$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Interest income, other	$ 4.8		$ 14.3
Derivative, fixed interest rate (as a percent)	3.78%		3.78%		3.54%
Basis points			30000.00%
Surplus Note | Build America Mutual
Guarantor Obligations [Line Items]
Interest expense	$ 4.8		$ 14.3
Combination One | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr			5
Combination Two | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr			3
US Treasury Rate | Surplus Note | BAM
Guarantor Obligations [Line Items]
Basis points			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]
2018	$ 2.5		$ 2.5
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2018	2.5		2.5
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2018	2.5		2.5
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2018	$ 2.4		$ 2.4
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