Acquisition and Disposals - Sale of Life Reinsurance narrative (Details) - USD ($) $ in Thousands				3 Months Ended		6 Months Ended
		May. 30, 2014	May. 01, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Future policy benefit reserves				$ 4,494,886		$ 4,494,886		$ 4,707,199
Future policy benefit reserves recoverable				4,054,705		4,054,705		4,265,678
Funds withheld on life retrocession arrangements (net of future policy benefit reserves recoverable)				915,116		915,116		$ 1,155,016
Underwriting profit (loss)				203,331	$ 135,075	342,538	$ 232,306
Comprehensive income (loss)				(628,996)	260,373	(408,284)	470,377
XL Re Life (SAC) Ltd
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of subsidiary stock, percentage		100.00%
Consideration received		$ 570,000
Quota share reinsurance percentage		100.00%
Future policy benefit reserves recoverable				4,100,000		4,100,000
Loss on disposal			$ 621,300
Funds withheld asset				5,000,000		5,000,000
Comprehensive income (loss)				0
Life insurance product line | XL Re Life (SAC) Ltd
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Future policy benefit reserves		$ 5,200,000
Future policy benefit reserves retained		400,000		400,000		400,000
Future policy benefit reserves recoverable		$ 4,800,000
Life Funds Withheld Assets | XL Re Life (SAC) Ltd
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Underwriting profit (loss)	[1]			0	0	603	0
Comprehensive income (loss)				$ 0	$ 0	$ 603	$ 0

[1] The underwriting profit of $0.6 million relates to a premium adjustment during the six months ended June 30, 2015 relating to the Life Retro Arrangements transaction. Excluding this transaction, the impact to comprehensive income relating to the Life Retro Arrangements was nil for the three and six months ended June 30, 2015.