Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified			3 Months Ended					9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 One Beacon Insurance Company [Member] Performance Share One Beacon Long Term Incentive Plan	Sep. 30, 2013 One Beacon Insurance Company [Member] One Beacon Long Term Incentive Plan Performance Share		Sep. 30, 2012 One Beacon Insurance Company [Member] One Beacon Long Term Incentive Plan Performance Share		Sep. 30, 2013 One Beacon Insurance Company [Member] One Beacon Long Term Incentive Plan Performance Share		Sep. 30, 2012 One Beacon Insurance Company [Member] One Beacon Long Term Incentive Plan Performance Share		Sep. 30, 2012 One Beacon Insurance Company [Member] One Beacon Long Term Incentive Plan Performance Share Performance cycle 2009-2011
Share-based compensation arrangement by share-based payment award
Vesting Period			3 years
Target Performance Shares Outstanding
Beginning of period (in shares)				489,867		560,577		563,190		642,667
Shares paid or expired (in shares)				0	[1]	0	[1]	(238,658)	[1]	(258,901)	[1]
New grants (in shares)				0		0		179,000		181,290
Assumed forfeitures and cancellations				0	[2]	(3,354)	[2]	(13,665)	[2]	(7,833)	[2]
Ending of period (in shares)				489,867		557,223		489,867		557,223
Expense Amortized
Accrued expense at the beginning of period	$ 161.6	$ 159.0		$ 2.4		$ 3.1		$ 1.2		$ 9.7
Payments and deferrals				0	[1]	0	[1]	0	[1]	(7.8)	[1]
Assumed forfeitures and cancellations				0	[2]	0	[2]	(0.1)	[2]	0	[2]
Expense recognized				0.1		(2.2)		1.4		(1.0)
Accrued expense at the end of the period	161.6	159.0		2.5		0.9		2.5		0.9
Percentage of target payout - minimum actual												138.60%
Additional compensation cost that would be recognized if all outstanding performance shares vested				$ 3.1				$ 3.1

[1] There were no payments made in 2013 for the 2010-2012 performance cycle; those performance shares did not meet the threshold performance goals and expired. OneBeacon performance share payments in 2012 for the 2009-2011 performance cycle were at 138.6% of target. Amounts include deposits into OneBeacon’s deferred compensation plan.

[2] Amounts include changes in assumed forfeitures, as required under GAAP.