Income Taxes (Deferred Tax Inventory) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred income tax assets related to:
U.S. federal net operating and capital loss carryforwards	$ 186.6	$ 152.3
Unearned premiums	37.5	38.3
Non-U.S. net operating loss carryforwards	33.8	33.1
Loss reserve discount	26.8	41.4
Tax credit carryforwards	16.9	13.4
Fixed assets	1.7	2.3
Accrued interest	1.3	5.0
Other items	3.0	2.8
Total gross deferred income tax assets	371.8	348.1
Less: valuation allowances	(156.8)	(140.5)
Total net deferred income tax assets	215.0	207.6
Deferred income tax liabilities related to:
Deferred acquisition costs	34.9	32.0
Net unrealized investment gains	28.2	31.1
Noncurrent deferred tax liabilities	3.5	2.5
Investment basis difference	13.8	17.0
Other items	0.5	0.1
Total deferred income tax liabilities	109.2	93.0
Net deferred tax asset	105.8	114.6
OneBeacon Runoff
Deferred income tax assets related to:
Runoff Transaction	12.6	12.6
Purchase Accounting [Member]
Deferred income tax liabilities related to:
Noncurrent deferred tax liabilities	9.3	0.0
Members Surplus Contributions [Member]
Deferred income tax liabilities related to:
Noncurrent deferred tax liabilities	19.0	10.3
Incentive Compensation [Member]
Deferred income tax assets related to:
Compensation	45.4	40.2
Deferred Compensation, Share-based Payments [Member]
Deferred income tax assets related to:
Compensation	6.2	$ 6.7
UNITED STATES
Deferred income tax assets related to:
Tax credit carryforwards	$ 14.6