Income Taxes (Deferred Tax Inventory) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred income tax assets related to:
Non-U.S. net operating loss carryforwards	$ 509.5	$ 503.7
U.S. federal net operating and capital loss carryforwards	165.2	102.7
U.S. federal net operating and capital loss carryforwards	88.6	114.2
Unearned premiums	45.5	41.2
Incentive compensation	36.8	43.4
Tax credit carryforwards	17.0	13.5
Deferred compensation	10.4	11.4
Pension and benefit accruals	10.3	8.0
Accrued interest	8.0	8.0
Fixed assets	2.8	(0.1)
Other items	7.7	6.4
Total gross deferred income tax assets	951.1	862.8
Less: valuation allowances	(254.0)	(232.6)
Total net deferred income tax assets	697.1	630.2
Deferred income tax liabilities related to:
Safety reserve	326.7	369.6
Deferred acquisition costs	54.2	34.8
Net unrealized investment gains	49.8	48.8
Investment basis difference	21.0	(0.9)
Purchase accounting	9.5	0.6
Other items	7.6	5.9
Total deferred income tax liabilities	468.8	458.8
Net deferred tax asset	228.3	171.4
OneBeacon Runoff
Deferred income tax assets related to:
Sale of AutoOne	49.3	0
AutoOne
Deferred income tax assets related to:
Sale of AutoOne	$ 0	$ 10.4