Acquisition and Disposals - Acquisition consideration (Details) - May. 01, 2015 £ / shares in Units, $ / shares in Units, £ in Thousands, $ in Thousands		USD ($) shares $ / shares	GBP (£) shares £ / shares
Business Acquisition [Line Items]
Closing price per XL share on April 30, 2015 | $ / shares	[1]	$ 37.08
Catlin Group Limited
Business Acquisition [Line Items]
Shares outstanding (including dilutive effects of warrants) that will receive cash and share consideration		384,118,000
Exchange ratio per the merger agreement		0.13	0.13
Share issuance to Catlin shareholders		49,935,000	49,935,000
Share issuance consideration | $		$ 1,851,601
Cash price component, per share in GBP | £ / shares			£ 3.88
Foreign exchange rate: GBP/USD		1.5349
Cash consideration		$ 2,287,579	£ 1,490,377
Total acquisition consideration | $		$ 4,139,180

[1] The closing market price of XL Shares on the Acquisition Date represents the fair value of XL shares issued as part of the Acquisition Consideration.