Losses and Loss Expenses - Asbestos and Environmental Exposures Discussion (Details) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2006	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011 claims
Loss And Loss Expenses Disclosure [Line Items]
Losses acquired through portfolio transfer, amount	$ 40.2
Combined aggregate limit on reserves acquired	60.0
Reserves for incurred but not reported losses, net of reinsurance		49.3	48.6	52.2
Net asbestos and environmental reserves to total reserves, percentage		1.00%	1.00%
Asbestos and environmental claims
Loss And Loss Expenses Disclosure [Line Items]
Losses acquired through portfolio transfer, amount	$ 18.3
Claim files due to precautionary claim notices, percentage		46.00%	37.00%	42.00%
Environmental claims
Loss And Loss Expenses Disclosure [Line Items]
Total number of claims outstanding		459	338	354	362
Asbestos claims
Loss And Loss Expenses Disclosure [Line Items]
Total number of claims outstanding		1,399	1,097	1,073	1,038