Acquisitions and Disposals - Life Retro Arrangements Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	May. 30, 2014	Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Underwriting profit (loss)		$ 587,409		$ 585,907	$ 245,191
Comprehensive income (loss)		(797,842)		747,801	$ (783,363)
XL Re Life (SAC) Ltd [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of Subsidiary Stock, Percentage	100.00%
Disposal Group, Including Discontinued Operation, Consideration	$ 570,000
Quota Share Reinsurance Percentage	100.00%
Pro Forma Effect of Sale, After Tax Net Loss				621,300
Comprehensive income (loss)		0		0
Life Funds Withheld Assets [Member] | XL Re Life (SAC) Ltd [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Underwriting profit (loss)		605	[1]	11,649
Comprehensive income (loss)		$ 605		$ 11,649

[1] The underwriting profit of $0.6 million relates to a premium adjustment relating to the GreyCastle Life Retro Arrangements transaction, which was completed on May 30, 2014. Excluding this transaction, the impact to comprehensive income relating to the GreyCastle Life Retro Arrangements was nil for the years ended December 31, 2015 and 2014.