Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended	12 Months Ended				1 Months Ended	12 Months Ended										3 Months Ended	12 Months Ended		7 Months Ended	1 Months Ended		12 Months Ended			1 Months Ended		7 Months Ended	12 Months Ended	1 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Nov. 30, 2012 2012 OBH Senior Notes	Dec. 31, 2012 2012 OBH Senior Notes	Dec. 31, 2011 2012 OBH Senior Notes	Dec. 31, 2012 2003 OBH Senior Notes	Dec. 31, 2011 2003 OBH Senior Notes	Mar. 31, 2007 SIG Senior Notes	Dec. 31, 2012 SIG Senior Notes	Dec. 31, 2011 SIG Senior Notes	Dec. 31, 2010 SIG Senior Notes	Dec. 31, 2012 WTM Bank Facility	Dec. 31, 2011 WTM Bank Facility	Dec. 31, 2010 WTM Bank Facility	Aug. 12, 2011 WTM Bank Facility	Aug. 12, 2011 Prior W T M Bank Facility	Dec. 31, 2012 Old Lyme	Dec. 31, 2011 Old Lyme	Dec. 31, 2011 Sierra Note	Dec. 31, 2010 One Beacon Insurance Group Ltd Atlantic Specialty Note	Mar. 31, 2004 One Beacon Insurance Group Ltd Atlantic Specialty Note	Dec. 31, 2010 One Beacon Insurance Company Senior Notes 2003 OBH Senior Notes	Dec. 31, 2012 One Beacon US Holdings Inc. 2003 OBH Senior Notes	May 31, 2003 One Beacon US Holdings Inc. 2003 OBH Senior Notes	Dec. 31, 2012 One Beacon US Holdings Inc. 2003 OBH Senior Notes	Dec. 31, 2011 One Beacon US Holdings Inc. 2003 OBH Senior Notes	Dec. 31, 2010 One Beacon US Holdings Inc. 2003 OBH Senior Notes	Apr. 30, 2011 One Beacon US Holdings Inc. Senior Notes 2003 OBH Senior Notes	Jun. 30, 2010 One Beacon US Holdings Inc. Senior Notes 2003 OBH Senior Notes	Dec. 31, 2010 One Beacon US Holdings Inc. Senior Notes 2003 OBH Senior Notes	Dec. 31, 2012 One Beacon US Holdings Inc. Senior Notes 2003 OBH Senior Notes	Sep. 30, 2010 Sirius Internation Group Ltd Sierra Note	Mar. 31, 2004 Sirius Internation Group Ltd Sierra Note	Dec. 31, 2010 Sirius Internation Group Ltd Sierra Note	Dec. 31, 2008 Sirius Internation Group Ltd Sierra Note	Dec. 31, 2007 Sirius Internation Group Ltd Sierra Note	Dec. 31, 2005 Sirius Internation Group Ltd Sierra Note	Dec. 31, 2012 Sirius Internation Group Ltd Sierra Note	Sep. 15, 2010 Sirius Internation Group Ltd Sierra Note
Debt Instrument
Debt instrument at face value				$ 275.0	$ 275.0	$ 0	$ 0	$ 269.9	$ 400.0	$ 400.0	$ 400.0														$ 700.0									$ 62.0
Debt	751.2	677.5			274.7	0	0	269.8		399.4	399.3		75.0	0				2.1	2.1
Percentage of par value at which debt was issued				99.90%					99.715%																99.70%
Issuance of debt, net of debt issuance costs	271.9	0	0	272.9					392.0																693.4
Debt issue costs				2.8					3.6																7.3
Underwriting discount				1.8					2.6																4.5
Interest rate (as a percent)				4.60%						6.375%												5.20%		5.875%		5.875%													4.00%
Effective yield (as a percent)				4.70%							6.50%		3.53%											6.00%		6.00%
Guarantee fee basis points per annum on outstanding principal amount of debt (as a percent)																								25.00%		25.00%
Expenses related to issuance, including underwriting fees				2.8					3.6																7.3
Interest expense, inclusive of amortization of issuance costs	44.8	55.2	57.3		1.8					26.2	26.2	26.2	1.7	3.1	1.1					5.5						15.1	20.5	29.5
Draw down of revolving line of credit	150.0	0	0										150.0
Aggregate principal amount of senior notes purchased and retired																							1.1						150.0	156.4	29.7	269.8
Aggregate purchase price of senior notes purchased and retired	275.9	161.6	197.3																				1.1	275.9					161.6	165.4	30.8
Loss on retirement of debt																								6.3					(12.0)	(9.6)	(1.2)
Write off of deferred issuance costs																								0.2
Loss from interest rate hedge recorded in other comprehensive income									2.4
Unamortized loss remaining in accumulated other comprehensive income										1.2
Total commitment under revolving credit facility																375.0	475.0					20.0
Percentage of principal amount of debt subject to upward or downward adjustments																			50.00%
Principal amount of purchase note																			6.0
Debt Instrument Term (in years)																			5 years			10 years												6 years
Amount of debt available for adjustment																																		58.0
Interest Paid	44.6	59.0	64.1
Increase (decrease) in the principal of the note issued																																		(29.0)	1.9	(5.2)	9.1	(22.8)
Undisputed amounts, principal and interest																																								42.8
Repayments of debt, principal	75.0	0	14.0										75.0								14.0												33.0
Repayments of debt, accrued interest																																	9.8
Reimbursement of debt repayment by Berkshire																																	36.7
Reimbursement of debt repayment by Berkshire, principal portion																																	33.0
Reimbursement of debt prepayment by Berkshire, accrued interest portion																																	3.7
Interest rate if outstanding principal amount of debt exceeds specified limit (as a percent)																						3.60%
Specified limit of outstanding principal amount of debt																						$ 15.0