Statutory Capital and Surplus (Details)	12 Months Ended			12 Months Ended				12 Months Ended		12 Months Ended			12 Months Ended					1 Months Ended	12 Months Ended			12 Months Ended							12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2011 Swedish kronor SEK	Dec. 31, 2011 OneBeacon USD ($)	Dec. 31, 2010 OneBeacon USD ($)	Dec. 31, 2009 OneBeacon USD ($)	Dec. 31, 2001 OneBeacon rating	Dec. 31, 2011 Sirius group Dividends resulting from reorganization USD ($)	Dec. 31, 2011 Other Operations USD ($)	Dec. 31, 2011 Regulated insurance operations OneBeacon USD ($)	Dec. 31, 2011 Unregulated insurance operations OneBeacon USD ($)	Dec. 31, 2011 OneBeacon Ltd. USD ($)	Dec. 31, 2011 OneBeacon Ltd. OneBeacon USD ($)	Dec. 31, 2011 OneBeacon Ltd. OneBeacon Ordinary dividends USD ($)	Dec. 31, 2011 OneBeacon Ltd. OneBeacon Special dividends USD ($)	Dec. 31, 2011 OneBeacon Ltd. OneBeacon Dividend received by White Mountains USD ($)	Dec. 31, 2011 Sirius International rating	Nov. 30, 2011 Sirius International Sirius group USD ($)	Dec. 31, 2011 Sirius International Sirius group USD ($)	Dec. 31, 2011 Sirius International Sirius group SEK	Dec. 31, 2011 Sirius International Sirius group Sweden USD ($)	Dec. 31, 2011 Sirius America Sirius group USD ($)	Dec. 31, 2010 Sirius America Sirius group USD ($)	Dec. 31, 2009 Sirius America Sirius group USD ($)	Dec. 31, 2011 Central National USD ($)	Dec. 31, 2010 Central National USD ($)	Dec. 31, 2011 Scandinavian Re Sirius group USD ($)	Dec. 31, 2011 White Shoals Re Sirius group USD ($)	Dec. 31, 2011 Sirius Group USD ($)	Dec. 31, 2011 Sirius Group USD ($)	Dec. 31, 2011 Sirius Group Ordinary dividends USD ($)	Dec. 31, 2011 WM Life Re Other Operations USD ($)	Dec. 31, 2011 WM Advisors USD ($)	Dec. 31, 2011 WM Advisors Other Operations USD ($)
Disclosure of statutory capital and surplus
Statutory net income (loss)				$ 119,600,000	$ 257,200,000	$ 210,500,000																$ 101,400,000	$ 70,200,000	$ 46,900,000	$ 100,000	$ 1,600,000
Restricted equity																					1,600,000,000
Statutory capital and surplus				1,000,000,000	1,100,000,000														2,100,000,000			533,700,000	742,600,000		13,400,000	15,000,000	27,600,000	15,400,000				26,400,000
Percentage of statutory surplus most recently reported to regulatory authorities that may be paid as dividends without prior approval of regulatory authorities																						10.00%
Percentage of prior year statutory surplus that may be paid as dividends without prior approval of regulatory authorities										10.00%
Amount available for dividend distributions without prior approval of regulatory authorities										103,000,000												53,400,000
Earned surplus																						75,500,000
Unassigned funds										700,000,000
Represents dividends paid by OneBeacon's unregulated insurance operating subsidiaries to their immediate parent													150,000,000
Distributions made								425,000,000			4,300,000			79,700,000	95,100,000	132,000,000			594,000,000										169,000,000		177,000,000		5,000,000
Net unrestricted cash, short-term investments and fixed maturity investments									1,554,300,000		183,000,000		16,900,000																	75,100,000				25,800,000
Number of ratings in A.M. Best ratings							15										15
Common equity securities and other long-term investments											30,000,000	30,000,000
Equity securities									316,600,000
Other long-term investments	301,300,000	372,100,000							33,400,000																					16,300,000
Increase in unrestricted statutory surplus due to reorganization																		436,300,000
Exchange rate			6.86
2010 pre-tax income transferred to parent company																			74,000,000
2011 pre-tax income intended to be transferred to parent company																			80,000,000
Unrestricted statutory surplus available for distribution																			514,900,000
Dividends intended to pay to the immediate parent																			177,000,000
Other long-term investments outside of Sirius America and Sirius International	301,300,000	372,100,000							33,400,000																					16,300,000
Safety Reserve
Amount of safety reserve																			1,400,000,000	9,600,000,000
Amount of Deferred Tax Liabilities on Safety Reserve	$ 369,600,000	$ 378,500,000																	$ 369,600,000
Swedish tax rate (as a percent)	26.30%