Derivative Instruments - Impact of life retro arrangements (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative [Line Items]
Underwriting profit (loss)		$ 162,192	$ 104,648	$ 431,920	$ 447,186
Net investment income		209,771	225,146	631,045	656,863
Foreign exchange (gains) losses		(1,695)	11,661	(54,614)	49,425
Other income and expenses	[1]	(3,271)	(4,420)	(8,324)	(12,292)
Net realized and unrealized gains (losses) on life retrocession embedded derivative		5,490	(7,903)	2,774	57,127
Net income (loss)		128,677	85,171	260,531	1,078,760
Change in adjustments related to future policy benefit reserves, net of tax		2,058	40,681	27,820	127,365
Foreign currency translation adjustments, net of tax		44,198	(22,394)	91,102	(20,714)
Comprehensive income (loss)		111,218	(121,092)	833,189	(529,376)
Life Funds Withheld Assets
Derivative [Line Items]
Net investment income		38,937	46,586	119,643	143,869
Net realized gains (losses) on investments sold		44,011	53,780	108,541	174,555
Net unrealized gains (losses) on investments, trading securities		41,270	(149)	165,653	(18,932)
Other-than-temporary impairments on investments		0	(2,023)	(2,598)	(10,110)
Net realized and unrealized gains (losses) on life retrocession embedded derivative		(225,610)	(126,140)	(691,432)	(116,333)
Change in net unrealized gains (losses) on investments, net of tax		77,242	(33,569)	206,480	(317,500)
XL Re Life (SAC) Ltd
Derivative [Line Items]
Comprehensive income (loss)				0
XL Re Life (SAC) Ltd | Life Funds Withheld Assets
Derivative [Line Items]
Underwriting profit (loss)	[2]	0	0	0	603
Net investment income		38,937	46,586	119,643	143,869
Net realized gains (losses) on investments sold		44,011	53,780	108,541	174,555
Net unrealized gains (losses) on investments, trading securities		41,270	(149)	165,653	(18,932)
Other-than-temporary impairments on investments		0	(2,023)	(2,598)	(10,110)
Foreign exchange (gains) losses		1,535	8,754	5,899	(5,932)
Other income and expenses		0	(121)	(170)	2,354
Net realized and unrealized gains (losses) on life retrocession embedded derivative		(225,610)	(126,140)	(691,432)	(116,333)
Net income (loss)		(99,857)	(19,313)	(294,464)	170,074
Change in net unrealized gains (losses) on investments, net of tax		77,242	(33,569)	206,480	(317,500)
Change in adjustments related to future policy benefit reserves, net of tax		2,058	40,681	27,820	127,365
Foreign currency translation adjustments, net of tax		20,557	12,201	60,164	20,664
Change In Other Comprehensive Income Life Retro Arrangements		99,857	19,313	294,464	(169,471)
Comprehensive income (loss)		$ 0	$ 0	$ 0	$ 603

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

[2] he underwriting profit of $0.6 million relates to a premium adjustment during the nine months ended September 30, 2015 relating to the GreyCastle Life Retro Arrangements transaction. Excluding this transaction, the impact to comprehensive income relating to the GreyCastle Life Retro Arrangements was nil for the nine months ended September 30, 2015.