Third-Party Reinsurance (OneBeacon) (Details) - USD ($) $ in Millions	3 Months Ended	5 Months Ended	6 Months Ended	7 Months Ended	12 Months Ended				60 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2020	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2020	Dec. 31, 2013	Dec. 31, 2012
Effects of Reinsurance [Line Items]
Reinsurance recoverable on unpaid losses	$ 186.5	$ 186.5		$ 186.5			$ 186.5	$ 161.7		$ 80.2	$ 107.3
OneBeacon
Effects of Reinsurance [Line Items]
Catastrophe Event Ratio			0.40%
OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance recoverable on paid losses	7.5	7.5		7.5			7.5
Reinsurance recoverable on unpaid losses	186.0	$ 186.0		186.0			$ 186.0
Property Catastrophe Program [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage							95.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation							$ 10.0
Property Catastrophe Program [Member] | Maximum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							130.0
Property Catastrophe Program [Member] | Minimum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							$ 20.0
Second Casualty Excess of Loss [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage							100.00%
Property Per-Risk, Acts of Terrorism | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage							100.00%
Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 100,000.0
Property Per-Risk, Acts of Terrorism | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							100.0
Property Per-Risk, Acts of Terrorism | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							3.0
Property Per Risk Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							100.0
Property Per Risk Reinsurance Program | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							3.0	$ 5.0
Individual Risk Facultative [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							100.0
Specialty Property | Property Catastrophe Reinsurance [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 34.0
Reinsurance Retention Policy, Reinsured Risk, Percentage							100.00%
Specialty Property | Property Catastrophe Reinsurance [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							$ 6.0
Inland and Ocean Marine | Marine Treaty Per Occurrence | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							2.5
Inland and Ocean Marine | Marine Treaty | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							60.0
Inland and Ocean Marine | Layer One | Property Per Risk Treaty | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							20.0
Inland and Ocean Marine | Layer One | Catastrophic Losses [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							2.5
Annual Aggregate Deductible to Reinsurance Retention Policy							5.0
Reinsurance retention policy, excess retention, amount reinsured							7.5
Health Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							10.0
Health Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							3.0
Health Insurance Product Line | HMO Provider Excess Reinsurance [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							5.0
Property, Liability and Casualty Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							11.0
Property, Liability and Casualty Insurance Product Line | Casualty Reinsurance Program | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 3.0
Workers compensation | Casualty Reinsurance Program | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage							100.00%
Workers compensation | Casualty Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							$ 10.0
Workers compensation | Casualty Reinsurance Program | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							2.0
Workers compensation | Dedicated Clash Treaty [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							10.0
Workers compensation | Dedicated Clash Treaty [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							60.0
Casualty and Surety [Member] | Dedicated Clash Treaty [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained							10.0
Casualty and Surety [Member] | Dedicated Clash Treaty [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							40.0
Healthcare Professional Liability [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							20.0
Healthcare Professional Liability [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							10.0
Other Casualty Business [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							21.0
Other Casualty Business [Member] | Second Casualty Excess of Loss [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured							11.0
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	$ 100.0
Reinsurance Retention Policy, Reinsured Risk, Percentage	100.00%
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	$ 50.0
Surety Product Line [Member] | Surety Reinsurance Program [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	5.0
Crop | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage		100.00%
Film Completion Bond Business [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				35.0
Film Completion Bond Business [Member] | OneBeacon | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				2.0
Film Completion Bond Business [Member] | Facultative Treaty Layer [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				60.0
OneBeacon Runoff | BBB | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	20.4	$ 20.4		20.4			20.4
Scenario, Forecast [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act					80.00%	84.00%
Decreasing Annual Percentage of Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act									1.00%
Retained Percentage Relating To Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act					20.00%	16.00%
Increased Annual Retained Percentage Relating to Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act						1.00%
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained						$ 120.0
Reinsurance Retention Policy, Reinsured Risk, Percentage						84.00%
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained						$ 100,000.0
Department of Treasury [Member] | Scenario, Forecast [Member] | Property and Casualty, Commercial Insurance Product Line [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured					$ 200.0	$ 120.0			$ 20.0
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	193.5	193.5		193.5			$ 193.5
Reinsurance recoverables, percentage of total							100.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AA | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	41.7	41.7		41.7			$ 41.7
Reinsurance recoverables, percentage of total							22.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | A | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	126.2	126.2		126.2			$ 126.2
Reinsurance recoverables, percentage of total							65.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | BBB | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables	$ 25.6	$ 25.6		$ 25.6			$ 25.6
Reinsurance recoverables, percentage of total							13.00%
Expiration March 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member] | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			$ 7,500.0
Reinsurance retention policy, excess retention, amount reinsured			5.0
Expiration March 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member] | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			5,000.0
Reinsurance retention policy, excess retention, amount reinsured			5.0
Expiration December 2015 [Member] | All Natural Perils [Member] | Industry Loss Warranty [Member]
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			15,000.0
Reinsurance retention policy, excess retention, amount reinsured			15.0
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member]
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured			30.0
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			10,000.0
Expiration December 2015 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			5,000.0
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member]
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured			7.5
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			7,500.0
Expiration March 2016 [Member] | Wind and earthquake [Member] | Industry Loss Warranty [Member] | Minimum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			5,000.0
Expiration June 2015 [Member] | Wind and flood [Member] | Industry Loss Warranty [Member]
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained			20,000.0
Reinsurance retention policy, excess retention, amount reinsured			$ 5.0