The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Amount of premium revenue under financial guarantee insurance contracts expected to be recognized in the sixth through tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

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The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

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Municipal Bond Guarantee Insurance (Details) $ in Millions	3 Months Ended								12 Months Ended
Municipal Bond Guarantee Insurance (Details) $ in Millions	Dec. 31, 2015 USD ($) Contract	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) Contract	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) Contract	Dec. 31, 2014 USD ($) Contract	Dec. 31, 2013 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	$ 258.1	$ (94.6)	$ (9.1)	$ 0.5	$ (65.7)	$ (35.5)	$ 33.6	$ 38.6	$ 154.9	$ (29.0)	$ 158.6
Net investment income (loss) - surplus note interest									0.0	$ 0.0	0.0
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Quarter	1.1								1.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Next Twelve Months	4.4								4.4
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two	4.3								4.3
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Three	4.1								4.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Four	3.8								3.8
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Five	3.6								3.6
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Six Through Ten	30.0								30.0
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Year Two	1.1								1.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Third Quarter	1.1								1.1
Financial Guarantee Insurance Contracts, Future Expected Premium Revenue to be Recognized, Fourth Quarter	$ 1.1								$ 1.1
Municipal bond contract \| Contract	3,103				1,750				3,103	1,750
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period									12 years 10 months	12 years 10 months
Municipal bond outstanding principal	$ 22,556.0				$ 12,362.5				$ 22,556.0	$ 12,362.5
Municipal bond outstanding interest	11,984.4				7,086.9				11,984.4	7,086.9
Financial Guarantee Insurance Contracts, Unearned Premium Revenue	$ 50.2				$ 27.6				$ 50.2	$ 27.6
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												503.0
Percentage of par value of policy reinsured	15.00%								15.00%
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%				88.40%	88.40%
BAM
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Surplus notes	$ 503.0								$ 503.0
Pre-tax loss									(47.3)	$ (40.5)	(78.6)
Net investment income (loss) - surplus note interest									(15.8)	(15.7)	(40.2)
HG Re
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Statutory capital and surplus	$ 460.8				$ 449.1				$ 460.8	$ 449.1
Collateral Held in Supplement Trust	136,200,000				120,000,000				136,200,000	120,000,000
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									$ 17.9	$ 18.3	38.1
Net investment income (loss) - surplus note interest									15.8	$ 15.7	$ 40.2
Surplus Note \| HG Global
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Interest Receivable	$ 90.2								$ 90.2