Statutory Capital and Surplus (Details) (USD $)	12 Months Ended			3 Months Ended	0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Jan. 02, 2013	Dec. 31, 2011
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	$ 3,996,600,000	$ 3,905,500,000
Amount available for dividend distributions without prior approval from regulatory authorities	500,000,000
Equity held in insurance and reinsurance subsidiaries	2,100,000,000
Distributions made	6,200,000
Fair value investments	5,991,400,000	6,766,500,000	6,950,700,000
Trading Securities	4,796,500,000
Dividend Capacity
Other long-term investments	408,200,000	288,900,000
Sirius Group
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	1,604,100,000	1,459,600,000
Unrestricted cash	20,400,000
Cash Dividends Paid to Parent Company	50,000,000
OneBeacon
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	(14,200,000)	100,100,000	82,000,000
Stockholders' Equity Attributable to Parent	788,400,000	830,600,000
Distributions made	80,000,000
Unrestricted cash	107,000,000
Other Operations
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	1,038,600,000	1,025,800,000
Unrestricted cash	275,100,000
Dividend Capacity
Other long-term investments	106,400,000
Unregulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made	4,800,000	17,300,000
Surplus notes	101,000,000
Unrestricted cash	78,100,000
Star & Shield Insurance Exchange [Member]
Statutory Accounting Practices [Line Items]
Surplus notes	17,000,000
Star & Shield Insurance Exchange [Member] | Other Operations
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	6,500,000
Statutory net income (loss)	(9,500,000)
Surplus required	2,800,000
Alstead [Member] | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	25,500,000
Surplus required	1,100,000
Central National [Member] | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus		9,300,000
Sirius Bermuda [Member] | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	2,400,000,000
Surplus required	1,000,000
Amount available for dividend distributions without prior approval from regulatory authorities	350,000,000
Percentage of prior year statutory surplus that may be paid as dividends without prior approval of regulatory authorities	15.00%
Woodridge [Domain] | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	8,300,000	8,400,000
Statutory net income (loss)	(200,000)	(100,000)	100,000
Surplus required	2,000,000
OneBeacon
Statutory Accounting Practices [Line Items]
Dividends available, net of noncontrolling interest	199,000,000
Sirius International | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	2,300,000,000
Surplus required	158,200,000
Restricted equity	1,400,000,000
Cash Dividends Paid to Parent Company	220,900,000
Statutory capital and surplus required, percent	125.00%
Maximum contributions per bankruptcy agreement	200,000,000
Ratio limitation	70.00%
Expense limitation	90,000,000
Dividend Capacity
Amount of safety reserve	1,300,000,000
Exchange rate	7.77
Deferred Tax Liabilities, Safety Reserve	295,700,000
Sirius International | Regulated Operation | Sirius Group
Statutory Accounting Practices [Line Items]
Unrestricted statutory surplus available for distribution	467,000,000
HG Global
Statutory Accounting Practices [Line Items]
Surplus notes	503,000,000	503,000,000
HG Global | HG Global-BAM
Dividend Capacity
Preferred stock, value	613,000,000
Ownership interest (as a percent)	96.90%
Preferred stock dividend rate	6.00%
HG Re | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	444,500,000
Collateral Held in Supplement Trust	402,600,000
Dividend Capacity
Accrued interest on surplus notes	44,300,000
BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	448,700,000
Surplus required	66,000,000
Subsidiaries
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	2,600,000,000
One Beacon Insurance Company | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made	700,500,000
Noncash distribution received	151,200,000
Payments of Capital Distribution		190,000,000
Proceeds from Contributions from Affiliates		35,000,000
Split Rock [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	122,600,000	96,400,000
Statutory net income (loss)	46,200,000	(38,700,000)
Distributions made	10,000,000
Payments of Capital Distribution				10,000,000
Proceeds from Contributions from Affiliates	135,100,000
White Mountains | OneBeacon
Statutory Accounting Practices [Line Items]
Distributions made		60,300,000
White Mountains | Other Operations
Statutory Accounting Practices [Line Items]
Proceeds from Contributions from Affiliates	15,000,000
Unrestricted cash	25,700,000
Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Surplus required	136,000,000
Payments of Capital Distribution	67,000,000	190,000,000
Atlantic Speciality Insurance Company (ASIC) [Member] | Regulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	721,500,000
Amount available for dividend distributions without prior approval from regulatory authorities	44,900,000
Statutory accounting practices earned surplus	87,800,000
Sirius America | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	620,600,000	548,400,000
Statutory net income (loss)	56,100,000	55,900,000	26,200,000
Surplus required	106,500,000
Statutory accounting practices earned surplus	81,900,000
Percentage of prior year statutory surplus that may be paid as dividends without prior approval of regulatory authorities		10.00%
Oakwood | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	29,400,000	20,700,000
Statutory net income (loss)	(800,000)	(900,000)	3,900,000
Surplus required	8,100,000
Ashmere [Member] | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	12,500,000	12,700,000
Statutory net income (loss)	(500,000)	(400,000)
Surplus required	2,000,000
Empire [Member] | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	10,500,000	10,700,000
Statutory net income (loss)	(600,000)	(10,400,000)
Surplus required	3,400,000
White Shoals Reinsurance Company | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	22,400,000	14,600,000
Surplus required	8,200,000	6,200,000
White Mountains Life Re | Other Operations
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	76,000,000	85,900,000
Surplus required	400,000
Olympus [Member] | Sirius Group
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	12,300,000
Surplus required	2,600,000
Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Interest rate (as a percent)	8.00%
basis points	3.00%
SWEDEN | Sirius Group
Dividend Capacity
Effective income tax			26.30%		22.00%
Preferred stocks | HG Global | HG Global-BAM
Dividend Capacity
Dividends Payable	96,200,000
Preferred stocks | White Mountains | HG Global | HG Global-BAM
Dividend Capacity
Dividends Payable	93,300,000
SEK
Dividend Capacity
Exchange rate	7.7737	6.4339	6.4973
SEK | Sirius International | Sirius Group
Dividend Capacity
Amount of safety reserve	10,400,000,000
Common Stock [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Trading Securities	89,400,000
Common Equities [Member] | Other Operations
Statutory Accounting Practices [Line Items]
Trading Securities	172,700,000
Retained earnings
Statutory Accounting Practices [Line Items]
Stockholders' Equity Attributable to Parent	3,011,400,000	2,802,300,000	2,542,700,000			2,789,700,000
Amount available for dividend distributions without prior approval from regulatory authorities	958,000,000
Retained earnings | Split Rock [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Amount available for dividend distributions without prior approval from regulatory authorities	18,800,000
Statutory Accounting Practices, Statutory Capital, Percent	15.00%
Retained earnings | Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory Accounting Practices, Statutory Capital, Percent	10.00%
U.S and Bermuda [Member] | OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	721,500,000	866,200,000
NAIC Risk-Based Capital [Member] | Ashmere [Member] | Sirius Group
Statutory Accounting Practices [Line Items]
Surplus required	100,000
NAIC Risk-Based Capital [Member] | Empire [Member] | Sirius Group
Statutory Accounting Practices [Line Items]
Surplus required	1,200,000
Surplus Note | Unregulated Operation | OneBeacon
Statutory Accounting Practices [Line Items]
Fair value investments	$ 65,100,000
Combination One [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Number of years of change in interest rate	5
Combination Two [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Number of years of change in interest rate	3
US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Interest rate (as a percent)	3.13%
Fixed interest rate [Member] | Surplus Note | BAM | HG Global-BAM
Dividend Capacity
Interest rate (as a percent)	8.00%