Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) $ in Millions			1 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended
	Feb. 24, 2016 shares	Feb. 24, 2015 shares	May 31, 2011 shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 2015 USD ($) shares	Jun. 30, 2015 USD ($) shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 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	$ 3.9		$ 0.0	$ 18.0
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)				395,872	390,950	612,500	382,722	612,500
Issued (in shares)	170,650	75,950		0	0		170,650	75,950
Vested (in shares)				0	0		(157,500)	(296,000)
Assumed forfeitures and cancellations				0	0		0	(1,500)
Ending of period (in shares)				395,872	390,950	390,950	395,872	390,950
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance | $				$ 3.5	$ 3.5	$ 3.5	$ 2.5	$ 3.5
Issued | $				0.0	0.0		2.3	1.1
Vested | $							0.0
Assumed forfeitures and cancellations | $				0.0	0.0		0.0	0.0
Expense recognized | $				(0.7)	(0.5)		(2.0)	(1.6)
Unamortized grant date fair value, ending balance | $				$ 2.8	$ 3.0	$ 3.5	$ 2.8	$ 3.0
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)				441,206	462,147	517,470	449,435	517,470
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
Unamortized grant date fair value
Assumed forfeitures and cancellations | $				$ 0.0	$ 0.0		$ 0.0	$ 0.0
Expense recognized | $				(0.3)	$ 0.3		(0.6)	$ (0.2)
Unamortized grant date fair value, ending balance | $				$ 2.3			$ 2.3
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)							233,461	215,698
Ending of period (in shares)				219,112	177,409		219,112	177,409
Unamortized grant date fair value
Expense recognized | $				$ (0.5)	$ (0.2)		$ (1.4)	$ (0.6)
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