Statutory Capital and Surplus (Details) (USD $)	12 Months Ended			12 Months Ended							12 Months Ended		12 Months Ended		12 Months Ended			12 Months Ended		12 Months Ended				12 Months Ended		12 Months Ended					12 Months Ended											12 Months Ended								0 Months Ended	12 Months Ended													12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Split Rock [Member]	Dec. 31, 2013 Sirius Group	Dec. 31, 2012 Sirius Group	Dec. 31, 2013 OneBeacon	Dec. 31, 2012 OneBeacon	Dec. 31, 2011 OneBeacon	Dec. 31, 2013 Other Operations	Dec. 31, 2012 Other Operations	Dec. 31, 2012 Regulated Operation OneBeacon	Dec. 31, 2013 Regulated Operation OneBeacon	Dec. 31, 2013 Unregulated Operation OneBeacon	Dec. 31, 2013 OneBeacon	Dec. 31, 2013 Sirius International	Dec. 31, 2012 Sirius International Sirius Group	Dec. 31, 2013 Sirius International Sirius Group	Dec. 31, 2012 Sirius International Regulated Operation Sirius Group	Dec. 31, 2013 Sirius International Regulated Operation Sirius Group	Dec. 31, 2013 HG Global	Dec. 31, 2013 HG Re	Dec. 31, 2013 BAM	Dec. 31, 2013 Subsidiaries	Dec. 31, 2013 One Beacon Insurance Company	Dec. 31, 2013 One Beacon Insurance Company Regulated Operation OneBeacon	Dec. 31, 2013 Split Rock [Member]	Dec. 31, 2012 White Mountains	Dec. 31, 2013 Atlantic Speciality Insurance Company (ASIC) [Member]	Dec. 31, 2012 Atlantic Speciality Insurance Company (ASIC) [Member] Regulated Operation OneBeacon	Dec. 31, 2013 Atlantic Speciality Insurance Company (ASIC) [Member] Regulated Operation OneBeacon	Dec. 31, 2013 Sirius America Sirius Group	Dec. 31, 2012 Sirius America Sirius Group	Dec. 31, 2011 Sirius America Sirius Group	Dec. 31, 2012 Sirius America Regulated Operation Sirius Group	Dec. 31, 2013 Central National Sirius Group	Dec. 31, 2012 Central National Sirius Group	Dec. 31, 2013 Woodridge Sirius Group	Dec. 31, 2012 Woodridge Sirius Group	Dec. 31, 2013 Oakwood Sirius Group	Dec. 31, 2012 Oakwood Sirius Group	Dec. 31, 2012 PICO Holdings Sirius Group	Dec. 31, 2013 Citation Sirius Group	Dec. 31, 2012 Citation Sirius Group	Dec. 31, 2013 Ashmere [Member] Sirius Group	Dec. 31, 2013 Empire [Member] Sirius Group	Dec. 31, 2013 White Shoals Reinsurance Company Sirius Group	Dec. 31, 2013 White Mountains Life Re	Dec. 31, 2013 White Mountains Advisors LLC	Dec. 31, 2013 Surplus Note BAM	Jan. 02, 2013 SWEDEN Sirius Group	Dec. 31, 2012 SWEDEN Sirius Group	Dec. 31, 2013 Preferred stocks HG Global	Dec. 31, 2013 Preferred stocks White Mountains HG Global	Dec. 31, 2013 SEK	Dec. 31, 2012 SEK	Dec. 31, 2011 SEK	Dec. 31, 2013 SEK Sirius International Sirius Group	Dec. 31, 2013 Common Stock [Member] OneBeacon	Dec. 31, 2013 Common Equities [Member] Other Operations	Dec. 31, 2013 Retained earnings	Dec. 31, 2012 Retained earnings	Dec. 31, 2011 Retained earnings	Dec. 31, 2010 Retained earnings	Dec. 31, 2013 Retained earnings Split Rock [Member]	Dec. 31, 2013 U.S and Bermuda [Member] OneBeacon	Dec. 31, 2012 U.S and Bermuda [Member] OneBeacon	Dec. 31, 2013 UNITED STATES OneBeacon
Statutory Accounting Practices [Line Items]
Statutory capital and surplus			$ 96,400,000			$ 866,200,000						$ 900,000,000					$ 2,500,000,000				$ 436,900,000	$ 469,000,000								$ 700,000,000	$ 548,400,000	$ 528,300,000			$ 9,300,000	$ 11,200,000	$ 8,400,000	$ 8,600,000	$ 20,700,000	$ 24,500,000		$ 11,900,000	$ 13,100,000	$ 12,700,000	$ 10,700,000	$ 14,600,000	$ 85,900,000																		$ 1,000,000,000	$ 900,000,000	$ 900,000,000
Payments of Capital Distribution																								190,000,000				190,000,000
Proceeds from Contributions from Affiliates																								35,000,000		135,100,000
Statutory net income (loss)						100,100,000	82,000,000	119,600,000																							55,900,000	26,200,000	101,400,000		(500,000)	1,700,000	(100,000)	100,000	(900,000)	3,900,000		(300,000)	(4,200,000)	(400,000)	(10,400,000)
Restricted equity																	1,700,000,000
Surplus required						164,700,000											132,400,000					66,000,000									113,900,000				1,500,000		0		2,200,000		6,300,000	100,000		4,600,000	1,400,000	6,200,000	700,000
Stockholders' Equity Attributable to Parent	3,905,500,000	3,731,800,000		1,459,600,000	1,559,700,000	830,600,000	763,100,000		1,025,800,000	802,800,000													2,800,000,000																																					2,802,300,000	2,542,700,000	2,789,700,000	2,175,600,000
Amount available for dividend distributions without prior approval from regulatory authorities	800,000,000																								86,600,000					23,900,000																														755,000,000				20,300,000
Statutory Accounting Practices, Statutory Capital, Percent																																																																15.00%
Dividends available, net of noncontrolling interest														246,000,000
Percentage of prior year statutory surplus that may be paid as dividends without prior approval of regulatory authorities											10.00%																		10.00%			10.00%
Unassigned funds												600,000,000
Cash Dividends Paid to Parent Company				250,000,000
Cash dividends declared to parent company by consolidated subsidiaries					75,000,000													75,000,000
Distributions made	6,200,000						80,200,000						17,300,000														60,300,000
Net unrestricted cash, short-term investments and fixed maturity investments				67,000,000		217,500,000			231,000,000				57,900,000																																			17,300,000
Trading Securities	5,347,300,000																																																									89,800,000	490,200,000
Pre-tax income transferred to parent company															101,900,000
Unrestricted statutory surplus available for distribution																			587,000,000
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries																		326,400,000																75,000,000
Equity held in insurance and reinsurance subsidiaries		2,000,000,000
Statutory accounting practices earned surplus																														94,600,000	33,000,000
Exchange rate																	6.43																																					6.4339	6.4973	6.8645
Effective income tax																																																		22.00%	26.30%
Other long-term investments	288,900,000	294,200,000		20,300,000					139,200,000
Safety Reserve
Amount of safety reserve																	1,600,000,000																																								10,400,000,000
Deferred Tax Liabilities, Safety Reserve																	357,200,000
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
Preferred stock, value																				613,000,000
Ownership interest (as a percent)																				97.30%
Preferred stock dividend rate																				6.00%
Dividends Payable																																																				55,200,000	53,700,000
Accrued interest on surplus notes																					35,000,000
Collateral Held in Supplement Trust																					$ 400,000,000
Interest rate (as a percent)																																																	8.00%