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Municipal Bond Guarantee (Details) $ in Millions	1 Months Ended	3 Months Ended
Municipal Bond Guarantee (Details) $ in Millions	Jul. 31, 2012 USD ($)	Mar. 31, 2017 USD ($) yr Contract	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) Contract
Guarantor Obligations [Line Items]
Pre-tax (loss) income		$ 37.9	$ 8.5
Net investment income (loss) - surplus note interest		$ 0.0	$ 0.0
BAM
Guarantor Obligations [Line Items]
Municipal bond contract \| Contract		5,191		4,807
Insured Financial Obligations, Remaining Weighted Average Contract Period		10 years 10 months	10 years 10 months
Municipal bond outstanding principal		$ 35,222.1		$ 33,057.3
Municipal bond outstanding interest		17,589.9		16,396.6
Municipal bond debt service outstanding		$ 52,812.0		49,453.9
Year of U.S. Treasury Rate \| yr		1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Twelve Months		$ 6.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		8.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Three		7.8
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Five		7.0
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Four		7.4
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Six Through Ten		63.2
Financial Guarantee Insurance Contracts, Unearned Premium Revenue		99.8		83.0
HG Global
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 594.5
Surplus Notes	503.0
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ 6.6
HG Global \| Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		96.90%
HG Global \| Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		88.40%
Build America Mutual
Guarantor Obligations [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ (12.2)
BAM
Guarantor Obligations [Line Items]
Pre-tax (loss) income		(12.2)	$ (7.6)
Net investment income (loss) - surplus note interest		$ (4.8)	(4.5)
HG Re
Guarantor Obligations [Line Items]
Statutory Accounting Practices, Statutory Capital and Surplus, Balance				$ 467.4
Percentage of par value of policy reinsured		15.00%
HG Global
Guarantor Obligations [Line Items]
Pre-tax (loss) income		$ 6.6	7.3
Net investment income (loss) - surplus note interest		4.8	$ 4.5
BAM Management
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 14.5
Surplus Note \| BAM
Guarantor Obligations [Line Items]
Interest Income, Other		$ 4.8
Derivative, Fixed Interest Rate		3.78%		3.54%
Surplus Note \| Build America Mutual
Guarantor Obligations [Line Items]
Interest expense		$ 4.8
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
US Treasury Rate [Member] \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Basis points		3.00%
Fixed interest rate [Member] \| Surplus Note \| BAM
Guarantor Obligations [Line Items]
Derivative, Fixed Interest Rate		8.00%
Fourth Quarter, year two [Member] \| BAM
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		$ 2.0
Third Quarter, year two [Member] \| BAM
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		2.0
Second Quarter, year two [Member] \| BAM
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		2.0
First Quarter, year two [Member] \| BAM
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two		$ 2.1