Acquisitions and Disposals - Impact of Life Retro Arrangements (Details) - USD ($) $ in Thousands		12 Months Ended
		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
Net investment income - Life Funds Withheld Assets		872,370		918,625	957,716
Exchange (gains) losses		(22,504)		37,568	28,243
Other income and expenses	[1]	(13,356)		(6,891)	(5,692)
Net realized and unrealized (gains) losses on derivative instruments		53,123		29,886	7,798
Net income (loss)		1,313,339		268,838	1,136,647
Change in adjustments related to future policy benefit reserves, net of tax		170,688		(400,456)	(44,660)
Change in cumulative translation adjustment - Life Funds Withheld Assets, net of tax		(25,315)		(25,729)	(54,858)
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]
Comprehensive income (loss)		0		0
Life Funds Withheld Assets [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net investment income - Life Funds Withheld Assets		187,489		129,575	0
Net realized gains (losses) on investments sold		223,272		5,067	0
Net unrealized gains (losses) on investments, Trading - Life Funds Withheld Assets		(27,734)		(9)	0
OTTI on investments - Life Funds Withheld Assets		(13,357)		(20,587)	0
Net realized and unrealized (gains) losses on derivative instruments		(151,691)		(488,222)	0
Change in net unrealized gains (losses) on investments, net of tax		(421,604)		274,083	$ 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	[2]	11,649
Net investment income - Life Funds Withheld Assets		187,489		129,575
Net realized gains (losses) on investments sold		223,272		5,067
Net unrealized gains (losses) on investments, Trading - Life Funds Withheld Assets		(27,734)		(9)
OTTI on investments - Life Funds Withheld Assets		(13,357)		(20,587)
Exchange (gains) losses		4,788		10,099
Other income and expenses		2,280		(1,610)
Net realized and unrealized (gains) losses on derivative instruments		(151,691)		(488,222)
Net income (loss)		225,652		(354,038)
Change in net unrealized gains (losses) on investments, net of tax		(421,604)		274,083
Change in adjustments related to future policy benefit reserves, net of tax		170,688		74,009
Change in cumulative translation adjustment - Life Funds Withheld Assets, net of tax		25,869		17,595
Total changes to other comprehensive income as a result of Life Retro Arrangements		(225,047)		365,687
Comprehensive income (loss)		$ 605		$ 11,649

[1] Net fee income and other includes operating expenses from the Company's loss prevention consulting services business.

[2] 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.