Statutory Capital and Surplus (Details)	12 Months Ended					12 Months Ended							12 Months Ended		12 Months Ended						12 Months Ended						12 Months Ended																		1 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2012 Sirius Group USD ($)	Dec. 31, 2011 Sirius Group USD ($)	Dec. 31, 2012 OneBeacon USD ($)	Dec. 31, 2011 OneBeacon USD ($)	Dec. 31, 2010 OneBeacon USD ($)	Dec. 31, 2012 OneBeacon Ordinary Dividend USD ($)	Dec. 31, 2012 OneBeacon Dividends Received by White Mountains USD ($)	Dec. 31, 2012 Sirius Group USD ($)	Dec. 31, 2012 Sirius Group SEK	Dec. 31, 2012 Other Operations USD ($)	Dec. 31, 2011 Other Operations USD ($)	Dec. 31, 2012 Regulated Operation OneBeacon USD ($)	Dec. 31, 2012 Unregulated Operation OneBeacon USD ($)	Dec. 31, 2012 Sirius International Sirius Group USD ($)	Dec. 31, 2012 Sirius International Sirius Group SWEDEN USD ($)	Dec. 31, 2012 Sirius International Sirius Group USD ($)	Dec. 31, 2012 Sirius International Sirius Group SEK	Dec. 31, 2012 Sirius International Regulated Operation Sirius Group USD ($)	Dec. 31, 2012 HG Global USD ($)	Dec. 31, 2012 HG Re USD ($)	Dec. 31, 2012 BAM USD ($)	Dec. 31, 2012 Subsidiaries USD ($)	Dec. 31, 2012 One Beacon Insurance Company Regulated Operation OneBeacon USD ($)	Dec. 31, 2012 Atlantic Speciality Insurance Company (ASIC) [Member] Regulated Operation OneBeacon USD ($)	Dec. 31, 2012 Sirius America Sirius Group USD ($)	Dec. 31, 2011 Sirius America Sirius Group USD ($)	Dec. 31, 2010 Sirius America Sirius Group USD ($)	Dec. 31, 2012 Sirius America Regulated Operation Sirius Group USD ($)	Dec. 31, 2012 Central National Sirius Group USD ($)	Dec. 31, 2011 Central National Sirius Group USD ($)	Dec. 31, 2012 American General Sirius Group USD ($)	Dec. 31, 2012 American General Property Sirius Group USD ($)	Dec. 31, 2012 PICO Holdings Sirius Group USD ($)	Dec. 31, 2012 Citation Sirius Group USD ($)	Dec. 31, 2012 White Shoals Reinsurance Company Sirius Group USD ($)	Dec. 31, 2012 White Mountains Life Re USD ($)	Dec. 31, 2012 White Mountains Advisors LLC Other Operations USD ($)	Dec. 31, 2012 Surplus Note BAM	Dec. 31, 2012 SWEDEN Sirius Group	Dec. 31, 2011 SWEDEN Sirius Group	Dec. 31, 2012 Preferred stocks HG Global USD ($)	May 31, 2007 Preferred stocks Sirius Internation Group Ltd	Dec. 31, 2012 Preferred stocks White Mountains HG Global USD ($)
Disclosure of statutory capital and surplus
Statutory capital and surplus						$ 900,000,000	$ 1,000,000,000								$ 900,000,000			$ 2,500,000,000				$ 412,000,000	$ 412,000,000	$ 483,700,000			$ 700,000,000	$ 528,300,000	$ 533,700,000			$ 11,200,000	$ 13,400,000	$ 8,600,000	$ 24,500,000	$ 6,300,000	$ 13,100,000	$ 15,500,000	$ 33,700,000
Statutory net income (loss)						82,000,000	119,600,000	257,200,000							329,900,000													26,200,000	101,400,000	70,200,000		1,700,000	100,000
Restricted equity																		1,600,000,000
Surplus required						207,500,000												109,200,000						65,000,000				127,800,000				1,700,000		100,000	2,500,000	200,000	900,000	7,000,000	400,000
Stockholders' Equity Attributable to Parent	3,731,800,000	4,087,700,000		1,559,700,000	1,264,300,000	763,100,000	826,700,000						802,800,000	1,996,700,000											2,900,000,000
Amount available for dividend distributions without prior approval from regulatory authorities	1,200,000,000																									329,900,000		15,000,000
Dividends available, net of noncontrolling interest	268,000,000
Statutory capital, balance																												528,300,000
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															700,000,000
Represents dividends paid by OneBeacon's unregulated insurance operating subsidiaries to their immediate parent											40,000,000										24,000,000										55,000,000
Cash dividends declared to parent company by consolidated subsidiaries											75,000,000										75,000,000
Cash dividends paid and declared to parent company by consolidated subsidiaries											115,000,000										99,000,000
Distributions made									80,000,000	60,000,000					173,100,000	4,900,000
Distribution of Insurance Subsidiary to Parent															34,000,000
Net unrestricted cash, short-term investments and fixed maturity investments				72,000,000		272,400,000							132,100,000			28,600,000																								18,400,000
Common equity securities and other long-term investments						33,300,000
Pre-tax income transferred to parent company																		82,000,000
Pre-tax income intended to be transferred to parent company																		110,000,000
Unrestricted statutory surplus available for distribution																	852,000,000
Equity held in insurance and reinsurance subsidiaries	1,700,000,000
Amount of Retained Earnings Available for Dividend without Approval	800,000,000
Increase in unrestricted statutory surplus due to reorganization																	436,300,000
Cash dividends inteded to paid to parent																	50,000,000
Statutory accounting practices earned surplus																												56,100,000
Exchange rate												6.50
Other long-term investments	294,200,000	301,300,000		18,000,000									77,600,000
Equity securities													540,400,000
Safety Reserve
Amount of safety reserve																			1,500,000,000	9,600,000,000
Safety reserve	326,700,000	369,600,000															326,700,000
Swedish tax rate (as a percent)																																										22.00%	26.30%
Safety reserve																	1,500,000,000
Statutory capital and surplus required, percent																	125.00%
Maximum contributions per bankruptcy agreement																	200,000,000
Ratio limitation																	70.00%
Expense limitation																	110,000,000
Preferred stock, value																						613,000,000
Ownership interest (as a percent)																						97.30%
Preferred stock dividend rate																						6.00%																							7.506%
Dividends Payable																																												16,800,000		16,300,000
Accrued interest on surplus notes																							12,000,000
Collateral Held in Supplement Trust																							400,000,000
Interest rate (as a percent)																																									8.00%
Accrued interest receivable on surplus notes																						18,400,000
Cash dividends declared and paid	$ 6,600,000	$ 8,000,000	$ 8,800,000										$ 6,600,000