Municipal Bond Guarantee (Details) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) Contract	Mar. 31, 2017	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) yr Contract	Jun. 30, 2016 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2016 USD ($) Contract	Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items]
Pre-tax income (loss)	$ (0.4)		$ (25.1)	$ 1.0	$ (54.8)
Net investment income (loss) - 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	$ 6.7			$ 13.3
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]
Surplus notes	$ 500.0			$ 500.0
Year of U.S. Treasury rate | yr				1
Municipal bond contract | Contract	5,564			5,564			4,807
Remaining weighted average contract period outstanding (in years)				10 years 11 months	10 years 10 months
Municipal bond outstanding principal	$ 37,619.7			$ 37,619.7			$ 33,057.3
Municipal bond outstanding interest	19,013.0			19,013.0			16,396.6
Municipal bond debt service outstanding	56,632.5			56,632.5			49,453.9
July 1, 2017 - December 31, 2017	4.6			4.6
2018	9.0			9.0
2019	8.6			8.6
2020	8.2			8.2
2021	7.8			7.8
2022 and thereafter	71.7			71.7
Total gross unearned insurance premiums	109.9			109.9			$ 82.9
Build America Mutual
Guarantor Obligations [Line Items]
Income (loss) from continuing operations before income taxes, domestic	(11.5)			(23.7)
HG Global
Guarantor Obligations [Line Items]
Pre-tax income (loss)	6.7		5.9	13.3	$ 13.2
Net investment income (loss) - surplus note interest	4.7		4.4	9.5	8.9
HG Re
Guarantor Obligations [Line Items]
Statutory capital and surplus, balance	469.7			469.7
Percentage of par value of policy reinsured		15.00%
BAM
Guarantor Obligations [Line Items]
Pre-tax income (loss)	(11.5)		(9.0)	(23.7)	(16.6)
Net investment income (loss) - surplus note interest	(4.7)		$ (4.4)	(9.5)	$ (8.9)
BAM Management
Guarantor Obligations [Line Items]
Partners capital contribution						$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Interest income, other	$ 4.7			$ 9.5
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	$ 4.7			$ 9.5
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	$ 2.3			$ 2.3
Second Quarter, year two | BAM
Guarantor Obligations [Line Items]
2018	2.3			2.3
Third Quarter, year two | BAM
Guarantor Obligations [Line Items]
2018	2.2			2.2
Fourth Quarter, year two | BAM
Guarantor Obligations [Line Items]
2018	2.2			2.2
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			$ 300.0