Derivative Instruments - Impact of Life Retro Arrangements (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Underwriting profit (loss)		$ 547,325	$ 587,409	$ 585,907
Net investment income		827,133	872,370	918,625
Exchange (gains) losses		(9,578)	22,504	(37,568)
Other Operating Income (Expense), Net	[1]	(13,315)	(13,356)	(6,891)
Net realized and unrealized gains (losses) on derivative instruments		2,521	53,123	29,886
Net income (loss)		570,145	1,313,339	268,838
Cumulative adjustments recorded to value of future policy benefit reserves, amount		62,295	170,688	(400,456)
Foreign currency translation adjustments		95,473	(25,315)	(25,729)
Other Comprehensive Income (Loss), Net of Tax		28,930	(797,842)	747,801
Life Funds Withheld Assets
Derivative [Line Items]
Net investment income		154,751	187,489	129,575
Net realized gains (losses) on investments sold		152,589	223,272	5,067
Net unrealized gains (losses) on investments, Trading - Life Funds Withheld Assets		109,458	(27,734)	(9)
Other-than-temporary impairments (OTTI) on investments		(2,598)	(13,357)	(20,587)
Net realized and unrealized gains (losses) on derivative instruments		(540,090)	(151,691)	(488,222)
Change in net unrealized gains (losses) on investments, net of tax		(4,502)	(421,604)	274,083
XL Re Life (SAC) Ltd [Member]
Derivative [Line Items]
Other Comprehensive Income (Loss), Net of Tax		0	0
XL Re Life (SAC) Ltd [Member] | Life Funds Withheld Assets
Derivative [Line Items]
Underwriting profit (loss)		0	605	11,649
Net investment income		154,751	187,489	129,575
Net realized gains (losses) on investments sold		152,589	223,272	5,067
Net unrealized gains (losses) on investments, Trading - Life Funds Withheld Assets		109,458	(27,734)	(9)
Other-than-temporary impairments (OTTI) on investments		(2,598)	(13,357)	(20,587)
Exchange (gains) losses		(8,988)	4,788	10,099
Other Operating Income (Expense), Net		(154)	2,280	(1,610)
Net realized and unrealized gains (losses) on derivative instruments		(540,090)	(151,691)	(488,222)
Net income (loss)		(135,032)	225,652	(354,038)
Change in net unrealized gains (losses) on investments, net of tax		(4,502)	(421,604)	274,083
Cumulative adjustments recorded to value of future policy benefit reserves, amount		62,295	170,688	74,009
Foreign currency translation adjustments		77,239	25,869	17,595
Change In Other Comprehensive Income Life Retro Arrangements		135,032	(225,047)	365,687
Other Comprehensive Income (Loss), Net of Tax		$ 0	$ 605	$ 11,649

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