Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) $ in Millions	1 Months Ended	3 Months Ended
	May. 31, 2011 shares	Mar. 31, 2016 USD ($) shares	Mar. 31, 2015 USD ($) shares	May. 25, 2011 Installment
Share-based compensation arrangement by share-based payment award
Equity in earnings of unconsolidated affiliates, net of tax | $		$ 0.0	$ 7.3
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)		382,722	612,500
Issued (in shares)		170,650	75,950
Vested (in shares)		(157,500)	(296,000)
Assumed forfeitures and cancellations		0	(1,500)
Ending of period (in shares)		395,872	390,950
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance | $		$ 2.5	$ 3.5
Issued | $		2.3	1.1
Assumed forfeitures and cancellations | $		0.0	0.0
Expense recognized | $		(0.5)	(0.6)
Unamortized grant date fair value, ending balance | $		$ 4.3	$ 4.0
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock | Chief Executive Officer
Target Restricted Share Activity
Issued (in shares)	630,000
Unamortized grant date fair value
Number of Installments in which the Shares Vest | Installment				4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years	35,000
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share
Target Restricted Share Activity
Beginning of period (in shares)		449,435	517,470
Assumed forfeitures and cancellations		(4,079)	(28,920)
Ending of period (in shares)		441,206	462,147
Unamortized grant date fair value
Assumed forfeitures and cancellations | $		$ 0.0	$ 0.0
Expense recognized | $		(0.4)	$ (0.2)
Unamortized grant date fair value, ending balance | $		$ 3.1
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share | Chief Executive Officer
Unamortized grant date fair value
Annual reduction in performance shares	35,000
Years Performance Shares will be Forfeited	5 years
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock Units (RSUs) [Member]
Target Restricted Share Activity
Issued (in shares)		222,449	207,848
Ending of period (in shares)		198,928
Unamortized grant date fair value
Expense recognized | $		$ (0.4)	$ (0.1)
Restricted Share Vesting in February 2018 [Member] | OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock
Target Restricted Share Activity
Issued (in shares)		92,500
Restricted Share Vesting in February 2019 [Member] | OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock
Target Restricted Share Activity
Issued (in shares)		78,150