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	12 Months Ended			0 Months Ended	1 Months Ended
	Dec. 31, 2011	Dec. 31, 2013 Restricted Stock	May 31, 2011 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan	Dec. 31, 2013 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan	Dec. 31, 2012 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan	Dec. 31, 2011 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan	Mar. 01, 2012 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan Chief Executive Officer Installment	May 31, 2011 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan Chief Executive Officer	May 25, 2011 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan Chief Executive Officer Installment
Target Performance Share activity
Beginning of period (in shares)				927,000	630,000	0
Issued (in shares)				0	300,000	630,000	300,000	630,000
Vested (in shares)				(9,000)	(667)	0
Assumed forfeitures and cancellations (in shares)	0			(3,000)	(2,333)
Ending of period (in shares)				915,000	927,000	630,000
Beginning balance				$ 9.6	$ 7.7	$ 0
Issued				0	4.5	8.6
Vested		0				0
Assumed forfeitures and cancellations				0	0	0
Expense recognized				(3.1)	(2.6)	(0.9)
Ending balance				$ 6.5	$ 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