Derivative Instruments - Impact of life retro arrangements (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Derivative [Line Items]
Underwriting profit (loss)		$ 138,456	$ 172,037
Net investment income		200,532	205,886
Foreign exchange (gains) losses		(3,336)	(33,819)
Other income and expenses	[1]	2,215	(2,723)
Net realized and unrealized gains (losses) on life retrocession embedded derivative		(7,069)	(3,622)
Net income (loss)		213,849	83,028
Change in adjustments related to future policy benefit reserves, net of tax		5,132	17,035
Foreign currency translation adjustments, net of tax		41,938	(16,439)
Comprehensive income (loss)		129,428	407,941
Life Funds Withheld Assets
Derivative [Line Items]
Net investment income		33,364	41,560
Net realized gains (losses) on investments sold		34,474	34,416
Net unrealized gains (losses) on investments, trading securities		(1,406)	69,096
Other-than-temporary impairments on investments		0	(2,346)
Net realized and unrealized gains (losses) on life retrocession embedded derivative		(50,101)	(236,080)
Change in net unrealized gains (losses) on investments, net of tax		(20,108)	51,990
XL Re Life (SAC) Ltd | Life Funds Withheld Assets
Derivative [Line Items]
Underwriting profit (loss)		0	0
Net investment income		33,364	41,560
Net realized gains (losses) on investments sold		34,474	34,416
Net unrealized gains (losses) on investments, trading securities		(1,406)	69,096
Other-than-temporary impairments on investments		0	(2,346)
Foreign exchange (gains) losses		3,294	11,119
Other income and expenses		(70)	(146)
Net realized and unrealized gains (losses) on life retrocession embedded derivative		(50,101)	(236,080)
Net income (loss)		19,555	(82,381)
Change in net unrealized gains (losses) on investments, net of tax		(20,108)	51,990
Change in adjustments related to future policy benefit reserves, net of tax		5,132	17,035
Foreign currency translation adjustments, net of tax		(4,579)	13,356
Change In Other Comprehensive Income Life Retro Arrangements		(19,555)	82,381
Comprehensive income (loss)		$ 0	$ 0

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