Income Taxes (Deferred Tax Inventory) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Contingency [Line Items]
Valuation allowance	$ 33,600,000	$ 14,100,000	$ (128,200,000)
Deferred income tax assets related to:
Non-U.S. net operating loss carryforwards	520,600,000	511,900,000
U.S. federal net operating and capital loss carryforwards	148,600,000	164,800,000
Loss reserve discount	74,900,000	88,600,000
Incentive compensation	10,300,000	10,400,000
Unearned premiums	38,900,000	45,500,000
Tax credit carryforwards	23,100,000	17,000,000
Fixed assets	4,900,000	2,800,000
Accrued interest	1,100,000	8,000,000
Pension and benefit accruals	0	10,300,000
Other items	6,900,000	5,700,000
Total gross deferred income tax assets	907,400,000	951,100,000
Less: valuation allowances	(289,800,000)	(254,000,000)
Total net deferred income tax assets	617,600,000	697,100,000
Deferred income tax liabilities related to:
Net unrealized investment gains	44,600,000	54,200,000
Investment basis difference	3,000,000	21,000,000
Deferred acquisition costs	33,000,000	49,800,000
Other items	7,900,000	7,500,000
Total deferred income tax liabilities	461,700,000	468,800,000
Net deferred tax asset	155,900,000	228,300,000
OneBeacon Runoff
Deferred income tax assets related to:
Runoff Transaction	24,200,000	49,300,000
Safety Reserve [Member]
Deferred income tax liabilities related to:
Deferred Tax Liabilities, Gross, Noncurrent	357,200,000	326,700,000
Purchase Accounting [Member]
Deferred income tax liabilities related to:
Deferred Tax Liabilities, Gross, Noncurrent	7,200,000	9,500,000
Members Surplus Contributions [Member]
Deferred income tax liabilities related to:
Deferred Tax Liabilities, Gross, Noncurrent	5,400,000	100,000
Other Pension Plan, Postretirement or Supplemental Plans, Defined Benefit [Member]
Deferred income tax liabilities related to:
Deferred Tax Liabilities, Gross, Noncurrent	3,400,000	0
Incentive Compensation [Member]
Deferred income tax assets related to:
Incentive compensation	53,900,000	36,800,000
UNITED STATES
Deferred income tax liabilities related to:
Net deferred tax asset	193,100,000
Luxembourg
Deferred income tax liabilities related to:
Net deferred tax asset	315,700,000
UNITED KINGDOM
Deferred income tax liabilities related to:
Net deferred tax asset	3,300,000
SWEDEN
Deferred income tax liabilities related to:
Total deferred income tax liabilities	$ 356,200,000