Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Less: Discount on workers compensation reserves	$ (1.6)	$ (1.1)
Total unpaid loss and LAE reserves, net of reinsurance	1,192.7	1,203.8	$ 1,180.6	$ 974.1
Plus: Total Reinsurance recoverables on unpaid losses	172.9	186.0
Total unpaid loss and LAE reserves	1,365.6	$ 1,389.8	$ 1,342.2	$ 1,054.3
OneBeacon
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	1,153.0
Less: Discount on workers compensation reserves	(1.6)
Total unpaid loss and allocated LAE reserves, net of reinsurance	1,151.4
Plus: Unallocated LAE	41.3
Total unpaid loss and LAE reserves, net of reinsurance	1,192.7
Plus: Total Reinsurance recoverables on unpaid losses	$ 172.9
Percentage of quota share reinsurance agreement	100.00%
Total unpaid loss and LAE reserves	$ 1,365.6
OneBeacon | Investing, Financing and Corporate
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Plus: Total Reinsurance recoverables on unpaid losses	18.3
Total unpaid loss and LAE reserves	18.3
Property | Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	30.2
Property | Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	108.5
Property | OneBeacon | Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	30.2
Plus: Total Reinsurance recoverables on unpaid losses	40.9
Property | OneBeacon | Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	108.5
Plus: Total Reinsurance recoverables on unpaid losses	13.2
Casualty | Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	568.7
Casualty | Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	360.0
Casualty | OneBeacon | Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	568.7
Plus: Total Reinsurance recoverables on unpaid losses	64.0
Casualty | OneBeacon | Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	360.0
Plus: Total Reinsurance recoverables on unpaid losses	19.9
Other | Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	38.2
Other | Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	47.4
Other | OneBeacon | Specialty Products
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	38.2
Plus: Total Reinsurance recoverables on unpaid losses	16.0
Other | OneBeacon | Specialty Industries
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	47.4
Plus: Total Reinsurance recoverables on unpaid losses	$ 0.6