Municipal Bond Guarantee (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Jul. 31, 2012 USD ($)	Sep. 30, 2015 USD ($) Contract	Sep. 30, 2014 USD ($)	Jun. 30, 2015	Sep. 30, 2015 USD ($) Contract yr	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) Contract
Guarantor Obligations [Line Items]
Pre-tax (loss) income		$ (94.6)	$ (35.5)		$ (103.2)	$ 36.7
Net investment income (loss) - surplus note interest		$ 0.0	0.0		$ 0.0	0.0
BAM
Guarantor Obligations [Line Items]
Municipal bond contract | Contract		2,733			2,733		1,750
Insured Financial Obligations, Remaining Weighted Average Contract Period					12 years 8 months		12 years 10 months
Municipal bond outstanding principal		$ 19,841.0			$ 19,841.0		$ 12,362.5
Municipal bond outstanding interest		10,574.3			10,574.3		7,086.9
Municipal bond debt service outstanding		30,415.3			30,415.3		19,449.4
Financial Guarantee Insurance Contracts, Unearned Premium Revenue		43.2			$ 43.2		$ 27.6
Year of U.S. Treasury Rate | yr					1
HG Global
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 594.5
Surplus Notes	503.0
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ 5.2			$ 13.8
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%
Build America Mutual
Guarantor Obligations [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ (10.6)	(13.1)		$ (33.5)	(29.7)
HG Re
Guarantor Obligations [Line Items]
Statutory Accounting Practices, Statutory Capital and Surplus, Balance		458.3			458.3
HG Global
Guarantor Obligations [Line Items]
Pre-tax (loss) income		5.2			13.8
Net investment income (loss) - surplus note interest		4.0			11.9
HG Global | HG Global
Guarantor Obligations [Line Items]
Pre-tax (loss) income			3.8			13.4
Net investment income (loss) - surplus note interest			3.9			11.8
BAM Management
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Basis points				3.00%
Interest Income, Other		$ 4.0			$ 11.9
Derivative, Fixed Interest Rate		3.15%			3.15%		3.13%
Surplus Note | Build America Mutual
Guarantor Obligations [Line Items]
Interest income (expense), surplus notes		$ 4.0	$ 3.9		$ 11.9	$ 11.8
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
Financial Guarantee [Member]
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured					15.00%
Fixed interest rate [Member] | Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, Fixed Interest Rate		8.00%			8.00%