Third Party Reinsurance (Sirius Group) (Details)					12 Months Ended		12 Months Ended	2 Months Ended	12 Months Ended									0 Months Ended		12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2014 Sirius Group USD ($)	Dec. 31, 2014 OneBeacon USD ($)	Dec. 31, 2014 Faculative and Direct Property Portfolios Sirius Group USD ($)	Feb. 28, 2014 Faculative and Direct Property Portfolios Lloyds Syndicate 1945 [Member] USD ($)	Dec. 31, 2014 Faculative and Direct U.S. Catastophe Exposed Business Lloyds Syndicate 1945 [Member] USD ($)	Dec. 31, 2014 Non U.S and Non Japan Earthquake Related Exposure Sirius Group USD ($)	Dec. 31, 2014 Non U.S and Non Japan Earth Quake Related Exposure Partial Coverage Sirius Group USD ($)	Dec. 31, 2014 Aviation Reinsurance Sirius Group USD ($)	Dec. 31, 2014 Aviation Reinsurance Minimum Sirius Group USD ($)	Dec. 31, 2014 Aviation Reinsurance Maximum Sirius Group USD ($)	Dec. 31, 2014 Marine Yacht Portfolio, London Branch and Syndicate 1945 Sirius Group USD ($)	Dec. 31, 2014 Energy & Marine [Member] Sirius Group USD ($)	Dec. 31, 2014 Quota Share Reinsurance [Member] Sirius Group EUR (€)	Jan. 01, 2015 Subsequent Event Sirius Group	Jan. 01, 2015 Subsequent Event Direct Contingency [Member] Sirius Group USD ($)	Dec. 31, 2014 Hamburg and Stockholm [Member] Faculative and Direct Property Portfolios Lloyds Syndicate 1945 [Member] USD ($)	Dec. 31, 2014 Euro Member Countries, Euro Accident and Health Sirius Group EUR (€)	Dec. 31, 2014 United States of America, Dollars Accident and Health Sirius Group USD ($)	Dec. 31, 2014 Expiration March 2015 [Member] Wind and flood [Member] Industry Loss Warranty [Member] Minimum USD ($)	Dec. 31, 2014 Expiration March 2015 [Member] Wind and flood [Member] Industry Loss Warranty [Member] Maximum USD ($)	Dec. 31, 2014 Expiration December 2015 [Member] All Natural Perils [Member] Industry Loss Warranty [Member] USD ($)	Dec. 31, 2014 Expiration December 2015 [Member] Wind and earthquake [Member] Industry Loss Warranty [Member] USD ($)	Dec. 31, 2014 Expiration December 2015 [Member] Wind and earthquake [Member] Industry Loss Warranty [Member] Minimum USD ($)	Dec. 31, 2014 Expiration December 2015 [Member] Wind and earthquake [Member] Industry Loss Warranty [Member] Maximum USD ($)	Dec. 31, 2014 Expiration March 2016 [Member] Wind and earthquake [Member] Industry Loss Warranty [Member] USD ($)	Dec. 31, 2014 Expiration March 2016 [Member] Wind and earthquake [Member] Industry Loss Warranty [Member] Minimum USD ($)	Dec. 31, 2014 Expiration March 2016 [Member] Wind and earthquake [Member] Industry Loss Warranty [Member] Maximum USD ($)	Dec. 31, 2014 Expiration June 2015 [Member] Wind and flood [Member] Industry Loss Warranty [Member] USD ($)
Reinsurance Retention Policy [Line Items]
Percentage of Business Ceded					20.00%
Reinsurance Program Limit Per Cedant					$ 15,000,000
Reinsurance retention policy, amount retained							5,000,000	2,500,000	5,000,000	35,000,000	75,000,000				250,000	1,500,000			10,000,000	20,000,000	5,000,000	6,000,000	5,000,000,000	7,500,000,000	15,000,000,000		5,000,000,000	10,000,000,000		5,000,000,000	7,500,000,000	20,000,000,000
Insurance Market Loss													250,000,000	2,000,000,000
Reinsurance retention policy, excess retention, amount reinsured							15,000,000	2,500,000	5,000,000	40,000,000	17,500,000	29,500,000			14,750,000	16,000,000	1,000,000			15,000,000	10,000,000	12,000,000	5,000,000	5,000,000	15,000,000	30,000,000			7,500,000			5,000,000
Percentage of quota share reinsurance agreement												15.00%							20.00%
Aviation reinsurance program cover in first event industry loss warranties													350,000,000	1,000,000,000
Percentage of trade credit business ceded under quota share retrocession					20.00%													10.00%
Percentage of bond business ceded under renewed quota share retrocession					50.00%													25.00%
Direct contingency business ceded					30.00%
Percentage of property catastrophe reinsurance treaty reinstatement premiums					100.00%
Reinsurance recoverable on paid losses					11,400,000	12,200,000
Reinsurance recoverable on unpaid losses	$ 483,900,000	$ 428,100,000	$ 429,100,000	$ 2,507,300,000	$ 322,200,000	$ 161,600,000