Municipal Bond Guarantee Insurance (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($) Contract	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Contract	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) Contract	Dec. 31, 2015 USD ($) Contract	Dec. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Pre-tax (loss) income from continuing operations	$ (66.8)	$ 20.0	$ (2.1)	$ 8.5	$ 246.3	$ (86.9)	$ (5.1)	$ 3.0	$ (40.4)	$ 157.3	$ (29.4)
Net investment income									86.8	$ 60.8	59.5
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Quarter	1.9								1.9
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Twelve Months	7.3								7.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two	7.0								7.0
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Three	6.6								6.6
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Four	6.3								6.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Five	5.9								5.9
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Six Through Ten	49.9								49.9
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Second Quarter	1.8								1.8
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Third Quarter	1.8								1.8
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Fourth Quarter	$ 1.8								$ 1.8
Municipal bond contract | Contract	4,807				3,103				4,807	3,103
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period									10 years 10 months	12 years 10 months
Municipal bond outstanding principal	$ 33,057.3				$ 22,556.0				$ 33,057.3	$ 22,556.0
Municipal bond outstanding interest	16,396.6				11,984.4				16,396.6	11,984.4
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	83.0				50.2				83.0	50.2
White Mountains
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions												$ 594.5
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 300.0								$ 300.0			503.0
BAM Management
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Capital contributions												$ 14.5
Preferred stocks | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	96.90%								96.90%
Common Stock | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ownership interest (as a percent)	88.40%								88.40%
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 503.0								$ 503.0
Pre-tax (loss) income from continuing operations									(48.9)	(47.3)	(40.5)
Net investment income									6.8	4.2	5.7
HG Re
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Statutory capital and surplus	467.2				452.9				467.2	452.9
Collateral Held in Supplement Trust	$ 465.4				$ 441.0				$ 465.4	441.0
HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Percentage of par value of policy reinsured	15.00%								15.00%
Pre-tax (loss) income from continuing operations									$ 22.2	17.9	18.3
Net investment income									2.2	1.9	1.4
Surplus Note
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Net investment income									0.0	0.0	0.0
Surplus Note | BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Net investment income									(17.8)	(15.8)	(15.7)
Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Net investment income									17.8	$ 15.8	$ 15.7
Interest Income [Member] | Surplus Note | HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Accrued Liabilities	$ 108.0								$ 108.0