Derivative Instruments - Impact of life retro arrangements (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Derivative [Line Items]
Underwriting profit (loss)	$ 188,261		$ 97,691		$ 326,717		$ 269,728
Net investment income	208,674		215,388		409,206		421,274
Foreign exchange (gains) losses	(5,643)		(19,100)		(8,979)		(52,919)
Other income and expenses	(1,997)	[1]	(2,330)	[2]	218	[3]	(5,053)	[4]
Net realized and unrealized gains (losses) on life retrocession embedded derivative	(906)		906		(7,975)		(2,716)
Net income (loss)	310,043		48,826		523,892		131,854
Change in adjustments related to future policy benefit reserves, net of tax	3,437		8,727		8,569		25,762
Foreign currency translation adjustments, net of tax	(1,092)		63,343		40,846		46,904
Comprehensive income (loss)	76,191		314,030		205,619		721,971
Life Funds Withheld Assets
Derivative [Line Items]
Net investment income	31,439		39,146		64,803		80,706
Net realized gains (losses) on investments sold	29,778		30,114		64,252		64,530
Net unrealized gains (losses) on investments, trading securities	(22,319)		55,287		(23,725)		124,383
Other-than-temporary impairments on investments	0		(252)		0		(2,598)
Net realized and unrealized gains (losses) on life retrocession embedded derivative	(34,596)		(229,742)		(84,697)		(465,822)
Change in net unrealized gains (losses) on investments, net of tax	(11,956)		77,248		(32,064)		129,238
XL Re Life (SAC) Ltd | Life Funds Withheld Assets
Derivative [Line Items]
Underwriting profit (loss)	0		0		0		0
Net investment income	31,439		39,146		64,803		80,706
Net realized gains (losses) on investments sold	29,778		30,114		64,252		64,530
Net unrealized gains (losses) on investments, trading securities	(22,319)		55,287		(23,725)		124,383
Other-than-temporary impairments on investments	0		(252)		0		(2,598)
Foreign exchange (gains) losses	14,964		(6,755)		18,259		4,364
Other income and expenses	(19)		(24)		(90)		(170)
Net realized and unrealized gains (losses) on life retrocession embedded derivative	(34,596)		(229,742)		(84,697)		(465,822)
Net income (loss)	19,247		(112,226)		38,802		(194,607)
Change in net unrealized gains (losses) on investments, net of tax	(11,956)		77,248		(32,064)		129,238
Change in adjustments related to future policy benefit reserves, net of tax	3,437		8,727		8,569		25,762
Foreign currency translation adjustments, net of tax	(10,728)		26,251		(15,307)		39,607
Change In Other Comprehensive Income Life Retro Arrangements	(19,247)		112,226		(38,802)		194,607
Comprehensive income (loss)	$ 0		$ 0		$ 0		$ 0

[1] Net fee income and other includes operating expenses of $12.2 million from the Company's loss prevention consulting services business.

[2] Net fee income and other includes operating expenses of $13.2 million from the Company's loss prevention consulting services business.

[3] Net fee income and other includes operating expenses of $23.7 million from the Company's loss prevention consulting services business.

[4] Net fee income and other includes operating expenses of $24.2 million from the Company's loss prevention consulting services business.