Third Party Reinsurance (Sirius Group) (Details)					12 Months Ended																								12 Months Ended													2 Months Ended		12 Months Ended
	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2013 Sirius Group USD ($)		Dec. 31, 2012 Sirius Group USD ($)	Dec. 31, 2013 OneBeacon USD ($)		Dec. 31, 2012 OneBeacon USD ($)	Dec. 31, 2013 AAA Sirius Group USD ($)		Dec. 31, 2013 AA Sirius Group USD ($)		Dec. 31, 2013 AA OneBeacon USD ($)		Dec. 31, 2013 A Sirius Group USD ($)		Dec. 31, 2013 A OneBeacon USD ($)		Dec. 31, 2013 BBB Sirius Group USD ($)		Dec. 31, 2013 BBB OneBeacon USD ($)		Dec. 31, 2013 BBB or lower Sirius Group USD ($)		Dec. 31, 2013 Not Rated [Member] Sirius Group USD ($)		Dec. 31, 2013 Faculative and Direct Property Portfolios Sirius Group USD ($)	Dec. 31, 2013 Faculative and Direct Property Portfolios Lloyds Syndicate 1945 [Member] USD ($)	Dec. 31, 2013 Faculative and Direct U.S. Catastophe Exposed Business Lloyds Syndicate 1945 [Member] USD ($)	Dec. 31, 2013 Non U.S and Non Japan Earthquake Related Exposure Sirius Group USD ($)	Dec. 31, 2013 Non U.S and Non Japan Earth Quake Related Exposure Partial Coverage Sirius Group USD ($)	Dec. 31, 2013 ILW Sirius Group USD ($)	Dec. 31, 2013 ILW Sirius Group EUR (€)	Dec. 31, 2013 Aviation Reinsurance Sirius Group USD ($)	Dec. 31, 2013 Aviation Reinsurance Minimum Sirius Group USD ($)	Dec. 31, 2013 Aviation Reinsurance Maximum Sirius Group USD ($)	Dec. 31, 2013 Marine Yacht Portfolio, London Branch and Syndicate 1945 Sirius Group USD ($)	Dec. 31, 2013 Accident and Health Sirius Group USD ($)	Dec. 31, 2013 Accident and Health Sirius Group EUR (€)	Feb. 28, 2014 Subsequent Event Faculative and Direct Property Portfolios Lloyds Syndicate 1945 [Member] USD ($)	Feb. 28, 2014 Subsequent Event ILW Sirius Group USD ($)	Dec. 31, 2013 Hamburg and Stockholm [Member] Faculative and Direct Property Portfolios Lloyds Syndicate 1945 [Member] USD ($)
Reinsurance Retention Policy [Line Items]
Reinsurance retention policy, amount retained																													$ 5,000,000	$ 5,000,000	$ 5,000,000	$ 35,000,000	$ 75,000,000	$ 7,500,000			$ 150,000,000	$ 2,250,000,000	$ 300,000	$ 6,900,000	€ 5,000,000	$ 2,500,000		$ 20,000,000
Reinsurance retention level, event policy																																		6,900,000,000	5,000,000,000								7,500,000,000
Percentage of quota share reinsurance agreement																																				15.00%
Percentage of trade credit business ceded under quota share retrocession					20.00%
Percentage of bond business ceded under renewed quota share retrocession					50.00%
Aviation reinsurance program cover in first event industry loss warranties																																					350,000,000	1,000,000,000
Reinsurance retention policy, excess retention, amount reinsured																													15,000,000	10,000,000	5,000,000	40,000,000	15,800,000			30,500,000			9,800,000	13,800,000	10,000,000	2,500,000	10,000,000	15,000,000
Direct contingency business ceded					30.00%
Percentage of property catastrophe reinsurance treaty reinstatement premiums					100.00%
Reinsurance recoverable on paid losses					15,700,000
Reinsurance recoverable on unpaid losses	428,100,000	429,100,000	2,507,300,000	2,344,000,000	347,900,000
Reinsurance recoverables, gross	453,500,000	447,000,000			363,600,000	[1]	336,300,000	89,900,000		110,700,000	62,900,000	[1]	60,600,000	[1]			127,500,000	[1]			21,000,000	[1]			16,600,000	[1]	75,000,000	[1]
Reinsurance Recoverables, Collateral					37,600,000	[1]					0	[1]	4,500,000	[1]			1,600,000	[1]			0	[1]			0	[1]	31,500,000	[1]
Reinsurance recoverable on paid and unpaid losses					$ 326,000,000	[1]		$ 89,900,000	[2]		$ 62,900,000	[1]	$ 56,100,000	[1]	$ 26,400,000	[2]	$ 125,900,000	[1]	$ 56,100,000	[2]	$ 21,000,000	[1]	$ 7,400,000	[2]	$ 16,600,000	[1]	$ 43,500,000	[1]
Reinsurance recoverables, percentage of total					100.00%	[1]		100.00%	[2]		19.00%	[1]	17.00%	[1]	30.00%	[2]	39.00%	[1]	62.00%	[2]	7.00%	[1]	8.00%	[2]	5.00%	[1]	13.00%	[1]

[1] Standard & Poor’s ratings as detailed above are: “AAA” (Extremely strong), “AA” (Very strong), “A” (Strong), and “BBB+” and “BBB” (Adequate).

[2] Standard & Poor’s ratings as detailed above are: “AA” (Very strong), “A” (Strong) and “BBB+” (Adequate).