Acquisition and Disposals - Acquisition Narrative (Details)			3 Months Ended		3 Months Ended			0 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2015 Catlin Group Limited USD ($)	Mar. 30, 2015 Catlin Group Limited USD ($)	Mar. 31, 2015 Catlin Group Limited Operating expense USD ($)	Mar. 31, 2015 Catlin Group Limited Interest expense USD ($)	Jan. 08, 2015 Catlin Group Limited XL Cayman GBP (£)	May 01, 2015 Subsequent Event Catlin Group Limited USD ($)	May 01, 2015 Subsequent Event Catlin Group Limited GBP (£)	May 01, 2015 Subsequent Event Catlin Group Limited Cash on hand USD ($)	May 01, 2015 Subsequent Event Catlin Group Limited Proceeds from debt issuance USD ($)
Business Acquisition [Line Items]
Ordinary shares, par value (in dollars per share)	$ 0.01	$ 0.01
Cash price component, per share in GBP									£ 3.88
Exchange ratio per the merger agreement								0.13	0.13
Share issuance to Catlin shareholders								49,935,000	49,935,000
Cash consideration								$ 2,287,579,000	£ 1,490,377,000	$ 1,310,000,000	$ 980,600,000
Available letter of credit facilities - commitments							1,600,000,000
Face value of issued notes				1,000,000,000
Acquisition, integration and financing costs			$ 25,500,000		$ 10,500,000	$ 15,000,000