Retirement and Postretirement Plans (Cash Flow) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee savings plan
Defined Benefit Plan, Benefit Obligation	$ 23,800,000	$ 109,500,000	$ 119,700,000
(Unfunded) over funded status at end of year	(10,800,000)	30,300,000
OneBeacon Ltd.
Expected Benefit Payments
Expected future benefit payments, next twelve months	2,100,000
Expected future benefit payments, year two	2,100,000
Expected future benefit payments, year three	2,000,000
Expected future benefit payments, year four	2,000,000
Expected future benefit payments, year five	1,900,000
Expected future benefit payments, five fiscal years thereafter	8,400,000
Employee savings plan
Anticipated contribution to plan		2,100,000
OneBeacon
Employee savings plan
Assets Held-in-trust, Noncurrent	29,300,000	33,900,000
Withdraw from rabbi trust	5,500,000
Disability and Health Benefits [Member] | OneBeacon
Employee savings plan
Post-employment benefit liability	3,100,000	4,100,000
Death benefits [Member] | OneBeacon
Employee savings plan
Postemployment Benefits Liability	12,800,000	12,500,000
One Beacon Employee Stock Ownership Plan
Employee savings plan
Defined Contribution Plan, Cost Recognized	$ 5,500,000	5,700,000	4,400,000
Annual base contribution provided to participants expressed as percentage of salary	3.00%
Social security wage base amount maximum	$ 118,500
Variable contribution provided to participants (as percentage)	6.00%
One Beacon Employee Stock Ownership Plan | OneBeacon
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	6.00%
Defined Contribution Plan, Cost Recognized	$ 3,200,000	$ 3,000,000	$ 2,700,000
IRS | OneBeacon
Employee savings plan
Defined Benefit Plan Compensation LImits	$ 265,000