Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 3.9		$ 0.0	$ 18.0
Expense Amortized
Accrued expense at the beginning of period				140.7
Accrued expense at the end of the period	$ 124.2			$ 124.2		$ 140.7
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period				3 years
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Cycle 2013 to 2015
Expense Amortized
Percentage of target payout - minimum actual				24.30%
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Expense Amortized
Percentage of target payout - minimum actual						45.70%
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	441,206	462,147	517,470	449,435	517,470	517,470
Shares paid or expired (in shares)	0	0		(167,300)	(181,290)
New grants (in shares)	0	0		163,150	154,887
Assumed forfeitures and cancellations	0	0		(4,079)	(28,920)
Ending of period (in shares)	441,206	462,147	462,147	441,206	462,147	449,435
Expense Amortized
Accrued expense at the beginning of period	$ 1.0	$ 2.4	$ 3.4	$ 1.4	$ 3.4	$ 3.4
Payments and deferrals	0.0	0.0		(0.7)	(1.5)
Assumed forfeitures and cancellations	0.0	0.0		0.0	0.0
Expense recognized	0.3	(0.3)		0.6	0.2
Accrued expense at the end of the period	1.3	$ 2.1	$ 2.4	1.3	$ 2.1	$ 1.4
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 2.3			$ 2.3