Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended					1 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended					12 Months Ended			1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended	3 Months Ended	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 2012 OBH Senior Notes	Dec. 31, 2012 2012 OBH Senior Notes	Mar. 31, 2007 SIG Senior Notes	Dec. 31, 2013 SIG Senior Notes	Dec. 31, 2012 SIG Senior Notes	Dec. 31, 2011 SIG Senior Notes	Dec. 31, 2013 WTM Bank Facility	Dec. 31, 2013 WTM Bank Facility	Dec. 31, 2012 WTM Bank Facility	Aug. 12, 2011 WTM Bank Facility	Sep. 30, 2013 Prior W T M Bank Facility	Dec. 31, 2013 Prior W T M Bank Facility	Dec. 31, 2012 Prior W T M Bank Facility	Dec. 31, 2011 Prior W T M Bank Facility	Jan. 31, 2013 Prior W T M Bank Facility	Aug. 12, 2011 Prior W T M Bank Facility	Dec. 31, 2011 Old Lyme	Dec. 31, 2013 Old Lyme	Dec. 31, 2012 Old Lyme	Nov. 30, 2012 One Beacon US Holdings Inc. 2012 OBH Senior Notes	Dec. 31, 2013 One Beacon US Holdings Inc. 2012 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	Apr. 30, 2011 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, 2011 Sirius Internation Group Ltd Sierra Note	Dec. 31, 2011 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	Sep. 15, 2010 Sirius Internation Group Ltd Sierra Note
Debt Instrument
Debt instrument at face value				$ 275.0	$ 275.0	$ 400.0	$ 400.0	$ 400.0															$ 275.0		$ 700.0						$ 62.0
Debt	676.4	751.2		274.7	274.7		399.6	399.4		0	0	0			0	75.0		75.0		2.1	2.1	2.1
Percentage of par value at which debt was issued						99.715%																	99.90%		99.70%
Issuance of debt, net of debt issuance costs	0	271.9	0			392.0																	272.9		693.4
Debt issue costs						3.6																	2.8		7.3
Underwriting discount						2.6																	1.8		4.5
Interest rate (as a percent)						6.375%																	4.60%		5.875%						4.00%
Effective yield (as a percent)						6.50%				3.65%	3.65%				2.83%	3.53%							4.70%		6.00%
Interest expense on debt	42.5	44.8	55.2				26.2	26.2	26.2		0.3				2.9	1.7	3.1															5.5
Debt Instrument, Repurchase Amount																													269.8
Guarantee fee basis points per annum on outstanding principal amount of debt (as a percent)																									2.50%
Expenses related to issuance, including underwriting fees						3.6																	2.8		7.3
Repayments of lines of credit										50.0				150.0
Aggregate principal amount of senior notes purchased and retired																												150.0
Aggregate purchase price of senior notes purchased and retired	0	275.9	161.6																									161.6	275.9
Loss on retirement of debt																												(12.0)	(6.3)
Interest Expense																								13.0		16.9	20.5
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							1.0
Total commitment under revolving credit facility													425.0						375.0
Draw down of revolving line of credit	200.0	150.0	0							50.0				150.0		150.0
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											6 years
Amount of debt available for adjustment																															58.0
Interest Paid	42.6	44.6	59.0																											9.8
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	275.0	75.0	0													75.0														33.0
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