Third Party Reinsurance (Recoverable Amounts by Reinsurer) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on unpaid losses	$ 172.9	$ 186.0	$ 161.6	$ 80.2
Reinsurance recoverable on paid and unpaid losses	179.5	193.5
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid and unpaid losses	179.5	$ 193.5
OneBeacon | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Recoverables	$ 179.5
Reinsurance recoverables, percentage of total	100.00%
Minimum | OneBeacon | Property Catastrophe Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	$ 20.0
Maximum | OneBeacon | Property Catastrophe Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	130.0
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on unpaid losses	172.9
OneBeacon | Specialty Property
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	34.0
Reinsurance retention policy, amount retained	6.0
OneBeacon | Combined Healthcare [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	10.0
OneBeacon | Surety Reinsurance Program [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	45.0
Reinsurance retention policy, amount retained	5.0
OneBeacon | Property Catastrophe Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	110.0
Reinsurance retention policy, amount retained	20.0
OneBeacon | Property Per Risk Reinsurance Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	100.0
Reinsurance retention policy, amount retained	3.0
OneBeacon | HMO Provider Excess Reinsurance [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	5.0
OneBeacon | Inland and Ocean Marine
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	57.5
Reinsurance retention policy, amount retained	2.5
OneBeacon | Film Completion Bond Business [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	38.0
Reinsurance retention policy, amount retained	2.0
OneBeacon | Casualty Clash/ Workers Compensation Catastrophe [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	34.0
Reinsurance retention policy, amount retained	6.0
OneBeacon | Workers compensation
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	20.0
Reinsurance retention policy, amount retained	40.0
OneBeacon | Financial Institutions - Professional Liability Reinsurance [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	10.0
OneBeacon | Casualty 2nd layer
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained	11.0
OneBeacon | Other Casualty Business [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	8.0
Reinsurance retention policy, amount retained	3.0
OneBeacon | Workers compensation per occurrence [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	8.0
Reinsurance retention policy, amount retained	2.0
OneBeacon | Healthcare Professional Liability [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	7.0
Reinsurance retention policy, amount retained	3.0
OneBeacon | Maximum | Specialty Property
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	6.0
OneBeacon | Maximum | Surety Reinsurance Program [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	5.0
OneBeacon | Maximum | Property Catastrophe Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	20.0
OneBeacon | Maximum | Property Per Risk Reinsurance Program
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	3.0
OneBeacon | Maximum | HMO Provider Excess Reinsurance [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	5.0
OneBeacon | Maximum | Inland and Ocean Marine
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	7.0
OneBeacon | Maximum | Film Completion Bond Business [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	2.0
OneBeacon | Maximum | Casualty Clash/ Workers Compensation Catastrophe [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	6.0
OneBeacon | Maximum | Workers compensation
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	6.0
OneBeacon | Maximum | Financial Institutions - Professional Liability Reinsurance [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	5.0
OneBeacon | Maximum | Other Casualty Business [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	3.0
OneBeacon | Maximum | Workers compensation per occurrence [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	2.0
OneBeacon | Maximum | Healthcare Professional Liability [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	3.0
One Beacon Insurance Company
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on unpaid losses	18.3
AM Best A plus | OneBeacon | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Recoverables	$ 73.9
Reinsurance recoverables, percentage of total	41.00%
AM Best A minus | OneBeacon | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Recoverables	$ 79.7
Reinsurance recoverables, percentage of total	44.00%
AM Best B | OneBeacon | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Recoverables	$ 25.9
Reinsurance recoverables, percentage of total	15.00%