Employee Share-Based Incentive Compensation Plans (OneBeacon Restsricted Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended								1 Months Ended	0 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 One Beacon Restricted Stock	Dec. 31, 2011 One Beacon Restricted Stock	Dec. 31, 2012 One Beacon Long Term Incentive Plan	Dec. 31, 2011 One Beacon Long Term Incentive Plan	Dec. 31, 2010 One Beacon Long Term Incentive Plan	May 31, 2011 One Beacon Long Term Incentive Plan One Beacon Restricted Stock	Mar. 01, 2012 Chief Executive Officer One Beacon Long Term Incentive Plan One Beacon Restricted Stock Installment	May 31, 2011 Chief Executive Officer One Beacon Long Term Incentive Plan One Beacon Restricted Stock	May 25, 2011 Chief Executive Officer One Beacon Long Term Incentive Plan One Beacon Restricted Stock Installment
Target Performance Share activity
Beginning of period (in shares)				630,000	0	642,667	1,464,295	2,224,215
Issued (in shares)	32,160	27,250	19,750	300,000	630,000					300,000	630,000
Vested (in shares)				(667)	0
Assumed forfeitures and cancellations (in shares)				(2,333)	0	(1,866)	(80,378)	(141,017)
Ending of period (in shares)				927,000	630,000	563,190	642,667	1,464,295
Beginning balance				$ 7.7	$ 0
Issued				4.5	8.6
Vested				0	0
Assumed forfeitures and cancelations				0	0	0	(0.5)	(2.2)
Expense recognized				(2.6)	(0.9)	(0.8)	2.2	10.2
Ending balance				$ 9.6	$ 7.7
Number of installments in which the shares vest										2		4
Forfeited shares									35,000		35,000
Years performance shares will be forfeited											5 years