Third Party Reinsurance (One Beacon) (Details) (USD $) In Millions, unless otherwise specified					3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 OneBeacon	Dec. 31, 2013 OneBeacon	Mar. 31, 2014 Crop insurance [Member] OneBeacon	Mar. 31, 2014 Crop-hail insurance [Member] OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Retained Percentage					101.50%	100.00%
Reinsurance recoverable on paid losses	$ 15.2	$ 25.4	$ 3.0	$ 9.7
Reinsurance recoverable on unpaid losses	$ 425.2	$ 428.1	$ 80.1	$ 80.2
Reinsurance Retention Policy, Reinsured Risk, Percentage					48.50%	80.00%