Income Taxes (Deferred Tax Inventory) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets related to:
U.S. federal net operating and capital loss carryforwards	$ 189.3	$ 191.0
Unearned premiums	37.8	37.5
Non-U.S. net operating loss carryforwards	29.5	33.8
Loss reserve discount	18.1	26.8
Tax credit carryforwards	20.7	16.9
Accrued interest	2.2	1.3
Other items	4.0	8.6
Total gross deferred income tax assets	362.4	383.7
Less: valuation allowances	(146.6)	(165.4)
Total net deferred income tax assets	215.8	218.3
Deferred income tax liabilities related to:
Deferred acquisition costs	33.6	34.9
Net unrealized investment gains	2.9	28.4
Deferred Tax Liabilities, Deferred Expense, Reserves and Accruals	6.4	3.5
Deferred Tax Liabilities, Deferred Expense, Capitalized Software	4.7	4.5
Investment basis difference	4.1	15.2
Other items	0.3	0.0
Total deferred income tax liabilities	89.1	105.5
Net deferred tax asset	126.7	112.8
OneBeacon Runoff
Deferred income tax assets related to:
Runoff Transaction	12.6	12.6
Members Surplus Contributions [Member]
Deferred income tax liabilities related to:
Noncurrent deferred tax liabilities	30.0	19.0
Surplus Note
Deferred income tax liabilities related to:
Noncurrent deferred tax liabilities	7.1	0.0
Incentive Compensation [Member]
Deferred income tax assets related to:
Compensation	42.5	49.0
Deferred Compensation, Share-based Payments [Member]
Deferred income tax assets related to:
Compensation	5.7	$ 6.2
UNITED STATES
Deferred income tax assets related to:
Tax credit carryforwards	$ 18.0