Retirement and Postretirement Plans (Benefit Obligations) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in projected benefit obligation:
Projected benefit obligation at beginning of year		$ 109.5	$ 119.7
Service cost		0.2	0.9	$ 0.6
Interest cost		1.5	4.6	4.7
Settlement loss (gain) on projected benefit obligation	$ (1.8)	1.2	(1.8)
Assumption changes		(0.9)	(3.5)
Actuarial loss		0.4	0.9
Annuity contract purchases related to termination of Qualified Plan (3)		0.0	0.0	(0.3)
Benefits paid directly by OneBeacon		(2.1)	(2.2)
Projected benefit obligation at end of year	23.8	23.8	109.5	119.7
Change in plan assets:
Fair value of plan assets at beginning of year		139.8	146.0
Actual return on plan assets		6.3	7.5
(Unfunded) over funded status at end of year	(10.8)	(10.8)	30.3
Fair value of plan assets at end of year	13.0	13.0	139.8	146.0
Amounts recognized in the financial statements
Net balance sheet asset recorded in other assets	13.0	13.0	55.8
Net balance sheet liability recorded in other liabilities	(23.8)	(23.8)	(25.5)
Defined Benefit Plan, Amounts Recognized in Balance Sheet	(10.8)	(10.8)	30.3
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		(3.0)	0.0	$ 0.0
Projected Benefit Obligations [Member]
Amounts recognized in the financial statements
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		(1.2)
Benefit and Expenses Paid with Plan Assets [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		17.0	13.7
Contract Termination [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		0.0	4.6
Annuity Contract Purchases [Member]
Change in projected benefit obligation:
Annuity contract purchases related to termination of Qualified Plan (3)		(69.0)	0.0
Defined Benefit Plan, Other Costs		69.0	0.0
Qualified Plan [Member]
Change in plan assets:
Fair value of plan assets at beginning of year		139.4
Fair value of plan assets at end of year	12.8	12.8	139.4
Other Postretirement Benefit Plan, Defined Benefit [Member]
Amounts recognized in the financial statements
Net balance sheet liability recorded in other liabilities	23.8	23.8
Other Postretirement Benefit Plan, Defined Benefit [Member] | OneBeacon Ltd.
Defined Benefit Plan Disclosure [Line Items]
Amount set aside in irrevocable rabbi trust	20.6	20.6
Transfer of Assets [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		47.1	0.0
Other Expense [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		17.0	$ 13.7
Principal Financial Group [Member]
Amounts recognized in the financial statements
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		(3.0)
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Group Annuity Contracts	69.0	69.0
Change in plan assets:
Fair value of plan assets at end of year	13.0	13.0
Amounts recognized in the financial statements
Net balance sheet asset recorded in other assets	$ 13.0	13.0
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements		(3.0)
Defined Benefit Plan, Assets Transferred to (from) Plan		47.1
Qualified Plan [Member] | Other Expense [Member]
Change in projected benefit obligation:
Defined Benefit Plan, Other Costs		$ (17.0)