Third-Party Reinsurance (OneBeacon) (Details) (USD $) In Millions, unless otherwise specified					12 Months Ended			12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013 OneBeacon		Dec. 31, 2013 OneBeacon	Dec. 31, 2013 Casualty Reinsurance Program OneBeacon	Dec. 31, 2013 Property Per-Risk, Acts of Terrorism OneBeacon	Dec. 31, 2012 Property Per-Risk, Acts of Terrorism OneBeacon	Dec. 31, 2013 Property Per-Risk, Acts of Terrorism Maximum OneBeacon	Dec. 31, 2013 Property Per-Risk, Acts of Terrorism OneBeacon Maximum	Dec. 31, 2013 Property Per-Risk, Acts of Terrorism OneBeacon Minimum	Dec. 31, 2013 Property Catastrophe Reinsurance OneBeacon	Dec. 31, 2013 Property Catastrophe Reinsurance Minimum OneBeacon	Dec. 31, 2013 Property Catastrophe Reinsurance OneBeacon	Dec. 31, 2013 Property Catastrophe Reinsurance OneBeacon Maximum	Dec. 31, 2012 Property Catastrophe Reinsurance OneBeacon Maximum	Dec. 31, 2013 Property Per Risk Reinsurance Program OneBeacon	Dec. 31, 2013 Property Per Risk Reinsurance Program OneBeacon Maximum	Dec. 31, 2013 Property Per Risk Reinsurance Program OneBeacon Minimum	Dec. 31, 2013 Layer One Property Catastrophe Reinsurance OneBeacon	Dec. 31, 2013 Layer One Property Catastrophe Reinsurance Minimum OneBeacon	Dec. 31, 2013 Layer One Property Catastrophe Reinsurance OneBeacon Maximum	Dec. 31, 2013 Layer One Property Per Risk Reinsurance Program OneBeacon Maximum	Dec. 31, 2013 Layer One Property Per Risk Reinsurance Program OneBeacon Minimum	Dec. 31, 2013 Layer Two Property Catastrophe Reinsurance OneBeacon	Dec. 31, 2013 Layer Two Property Catastrophe Reinsurance OneBeacon Maximum	Dec. 31, 2013 Layer Two Property Catastrophe Reinsurance OneBeacon Minimum	Dec. 31, 2013 Layer Two Property Per Risk Reinsurance Program OneBeacon	Dec. 31, 2013 Layer Two Property Per Risk Reinsurance Program OneBeacon Minimum	Dec. 31, 2013 Layer Three Property Catastrophe Reinsurance OneBeacon	Dec. 31, 2013 Layer Three Property Catastrophe Reinsurance OneBeacon Maximum	Dec. 31, 2013 Layer Three Property Catastrophe Reinsurance OneBeacon Minimum	Dec. 31, 2013 Inland and Ocean Marine Marine Treaty Per Occurrence OneBeacon	Dec. 31, 2013 Inland and Ocean Marine Marine Treaty OneBeacon	Dec. 31, 2013 Inland and Ocean Marine Layer One Marine Treaty OneBeacon	Dec. 31, 2013 Inland and Ocean Marine Layer One Marine Treaty OneBeacon Maximum	Dec. 31, 2013 Inland and Ocean Marine Layer One Marine Treaty OneBeacon Minimum	Dec. 31, 2013 Inland and Ocean Marine Layer One Property Per Risk Treaty OneBeacon Maximum	Dec. 31, 2013 Inland and Ocean Marine Layer One Catastrophic Losses [Member] OneBeacon	Dec. 31, 2013 Catastrophic Losses [Member] Layer One OneBeacon	Dec. 31, 2013 Surety Underwriting Insurance Per Bond OneBeacon	Dec. 31, 2013 Surety Underwriting Insurance Per Bond OneBeacon Maximum	Dec. 31, 2013 Surety Underwriting Insurance Per Bond OneBeacon Minimum	Dec. 31, 2013 Health Insurance Product Line Casualty Reinsurance Program OneBeacon Maximum	Dec. 31, 2013 Health Insurance Product Line Casualty Reinsurance Program OneBeacon Minimum	Dec. 31, 2013 Health Insurance Product Line Layer One Casualty Reinsurance Program OneBeacon	Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Casualty Reinsurance Program OneBeacon Maximum	Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Casualty Reinsurance Program OneBeacon Minimum	Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Layer One Casualty Reinsurance Program OneBeacon	Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Layer Two Casualty Reinsurance Program	Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Layer Two Casualty Reinsurance Program OneBeacon	Dec. 31, 2013 Workers compensation Casualty Reinsurance Program OneBeacon	Dec. 31, 2013 Workers compensation Casualty Reinsurance Program OneBeacon Maximum	Dec. 31, 2013 Workers compensation Casualty Reinsurance Program OneBeacon Minimum	Dec. 31, 2013 AA OneBeacon		Dec. 31, 2013 A OneBeacon		Dec. 31, 2013 BBB OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance recoverable on paid and unpaid losses					$ 89.9	[1]																																																			$ 26.4	[1]	$ 56.1	[1]	$ 7.4	[1]
Reinsurance recoverables, percentage of total					100.00%	[1]																																																			30.00%	[1]	62.00%	[1]	8.00%	[1]
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act					85.00%
Reinsurance retention policy, amount retained								10.0	100.0		100,000.0				20.0	33.0	150.0	180.0																	2.0					20.0	2.0							5.0			5.0		11.0
Annual Aggregate Deductible to Reinsurance Retention Policy																																									1.5	5.0										3.0
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation														117.0
Reinsurance retention policy, excess retention, amount reinsured								60.0				100.0	10.0	130.0						100.0	10.0		20.0	30.0	40.0	20.0		70.0	30.0		40.0		150.0	70.0		60.0		7.0	2.0		5.0		60.0	30.0	5.0	20.0	5.0	5.0	21.0	5.0	6.0		10.0		10.0	1.0
Property Catastrophe Reinsurance Treaty Second Layer Co-Partipation Percentage																														10.00%
Property Catastrophe Reinsurance Treaty First Layer Co-Partipation Percentage																			5.00%																		60.00%
Reinsurance Retention Policy, Excess Retention, Percentage																						50.00%					10.00%					5.00%
Reinsurance Retention Policy, Reinsured Risk, Percentage										85.00%																																	100.00%					10.00%			10.00%		5.00%	100.00%
Reinsurance recoverable on paid losses							9.7
Reinsurance recoverable on unpaid losses	$ 428.1	$ 429.1	$ 2,507.3	$ 2,344.0			$ 80.2

[1] Standard & Poor’s ratings as detailed above are: “AA” (Very strong), “A” (Strong) and “BBB+” (Adequate).