Derivative Instruments - Impact of life retro arrangements (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Derivative [Line Items]
Underwriting profit (loss)	$ (1,235,734)		$ 162,192		$ (909,017)		$ 431,920
Net investment income	202,779		209,771		611,985		631,045
Foreign exchange (gains) losses	55,017		(1,695)		46,038		(54,614)
Other income and expenses	(2,019)	[1]	(3,271)	[2]	(1,801)	[3]	(8,324)	[4]
Net realized and unrealized gains (losses) on life retrocession embedded derivative	(20,434)		5,490		(28,409)		2,774
Net income (loss)	(1,073,886)		128,677		(549,994)		260,531
Change in adjustments related to future policy benefit reserves, net of tax	5,412		2,058		13,981		27,820
Foreign currency translation adjustments, net of tax	(1,609)		44,198		39,237		91,102
Comprehensive income (loss)	47,763		111,218		253,382		833,189
Life Funds Withheld Assets
Derivative [Line Items]
Net investment income	30,796		38,937		95,599		119,643
Net realized gains (losses) on investments sold	18,879		44,011		83,131		108,541
Net unrealized gains (losses) on investments, trading securities	(7,331)		41,270		(31,056)		165,653
Other-than-temporary impairments on investments	0		0		0		(2,598)
Net realized and unrealized gains (losses) on life retrocession embedded derivative	(31,662)		(225,610)		(116,359)		(691,432)
Change in net unrealized gains (losses) on investments, net of tax	(14,579)		77,242		(46,643)		206,480
XL Re Life (SAC) Ltd | Life Funds Withheld Assets
Derivative [Line Items]
Underwriting profit (loss)	0		0		0		0
Net investment income	30,796		38,937		95,599		119,643
Net realized gains (losses) on investments sold	18,879		44,011		83,131		108,541
Net unrealized gains (losses) on investments, trading securities	(7,331)		41,270		(31,056)		165,653
Other-than-temporary impairments on investments	0		0		0		(2,598)
Foreign exchange (gains) losses	8,543		1,535		26,801		5,899
Other income and expenses	0		0		(90)		(170)
Net realized and unrealized gains (losses) on life retrocession embedded derivative	(31,662)		(225,610)		(116,359)		(691,432)
Net income (loss)	19,225		(99,857)		58,026		(294,464)
Change in net unrealized gains (losses) on investments, net of tax	(14,579)		77,242		(46,643)		206,480
Change in adjustments related to future policy benefit reserves, net of tax	5,412		2,058		13,981		27,820
Foreign currency translation adjustments, net of tax	(10,058)		20,557		(25,364)		60,164
Change In Other Comprehensive Income Life Retro Arrangements	(19,225)		99,857		(58,026)		294,464
Comprehensive income (loss)	$ 0		$ 0		$ 0		$ 0

[1] Net fee income and other includes operating expenses of $13.5 million from the Company's loss prevention consulting services business.

[2] Net fee income and other includes operating expenses of $11.9 million from the Company's loss prevention consulting services business.

[3] Net fee income and other includes operating expenses of $37.2 million from the Company's loss prevention consulting services business.

[4] Net fee income and other includes operating expenses of $36.0 million from the Company's loss prevention consulting services business.