Third-Party Reinsurance (OneBeacon) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended			60 Months Ended
	Mar. 31, 2017	Dec. 31, 2020	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2020	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effects of Reinsurance [Line Items]
Reinsurance recoverable on paid losses				$ 6.6		$ 7.5
Reinsurance recoverable on unpaid losses				172.9		$ 186.0	$ 161.6	$ 80.2
OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance recoverable on paid losses				6.6
Reinsurance recoverable on unpaid losses				172.9
Property Per Risk Reinsurance Program | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				3.0
Reinsurance retention policy, excess retention, amount reinsured				100.0
Property Per Risk Reinsurance Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				3.0
Workers compensation | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				40.0
Reinsurance retention policy, excess retention, amount reinsured				20.0
Workers compensation | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				$ 6.0
Property Catastrophe Program | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Retention Policy, Reinsured Risk, Percentage				100.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation				$ 110.0
Property Catastrophe Program | Maximum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				130.0
Property Catastrophe Program | Minimum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				20.0
Property Catastrophe Program | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				20.0
Reinsurance retention policy, excess retention, amount reinsured				110.0
Property Catastrophe Program | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				20.0
Specialty Property | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				6.0
Reinsurance retention policy, excess retention, amount reinsured				34.0
Specialty Property | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				6.0
Film Completion Bond Business [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				2.0
Reinsurance retention policy, excess retention, amount reinsured				38.0
Film Completion Bond Business [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				2.0
Combined Healthcare [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				10.0
Inland and Ocean Marine | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				2.5
Reinsurance retention policy, excess retention, amount reinsured				57.5
Inland and Ocean Marine | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				7.0
Combined Healthcare/ Casualty | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				10.0
Combined Healthcare/ Casualty | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				3.0
Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				100,000.0
Surety Reinsurance Program [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				5.0
Reinsurance retention policy, excess retention, amount reinsured				45.0
Surety Reinsurance Program [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				5.0
Other Casualty Business [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				3.0
Reinsurance retention policy, excess retention, amount reinsured				8.0
Other Casualty Business [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				3.0
HMO Provider Excess Reinsurance [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				5.0
HMO Provider Excess Reinsurance [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				5.0
Healthcare Professional Liability [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				3.0
Reinsurance retention policy, excess retention, amount reinsured				7.0
Healthcare Professional Liability [Member] | OneBeacon | Maximum
Effects of Reinsurance [Line Items]
Reinsurance retention policy, excess retention, amount reinsured				3.0
Casualty 2nd layer | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance retention policy, amount retained				11.0
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%	83.00%
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%	17.00%
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			$ 140.0
Reinsurance Retention Policy, Reinsured Risk, Percentage			83.00%
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	$ 140.0		$ 20.0
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables				$ 179.5
Reinsurance recoverables, percentage of total				100.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AM Best A plus | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables				$ 73.9
Reinsurance recoverables, percentage of total				41.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AM Best A minus | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables				$ 79.7
Reinsurance recoverables, percentage of total				44.00%
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AM Best B | OneBeacon
Effects of Reinsurance [Line Items]
Reinsurance Recoverables				$ 25.9
Reinsurance recoverables, percentage of total				15.00%