Derivative Instruments - Impact of life retro arrangements (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Derivative [Line Items]
Underwriting profit (loss)	$ 105,163		$ 188,261		$ 219,251		$ 326,717
Net investment income	231,788		208,674		450,269		409,206
Foreign exchange gains (losses)	12,140		5,643		2,299		8,979
Other income and expenses	(3,806)	[1]	(1,997)	[2]	(9,548)	[3]	218	[4]
Net realized and unrealized gains (losses) on life retrocession embedded derivative	16,126		(906)		20,347		(7,975)
Net income (loss)	336,800		310,043		498,033		523,892
Change in adjustments related to future policy benefit reserves, net of tax	42,314		3,437		46,478		8,569
Foreign currency translation adjustments, net of tax	(28,296)		(1,092)		(30,728)		40,846
Comprehensive income (loss)	(260,991)		76,191		(620,214)		205,619
Life Funds Withheld Assets
Derivative [Line Items]
Net investment income	30,252		31,439		60,650		64,803
Net unrealized gains (losses) on investments, trading	(1,896)		(22,319)		(33,563)		(23,725)
Other-than-temporary impairments	0		0		(1,203)		0
Net realized and unrealized gains (losses) on life retrocession embedded derivative	(36,120)		(34,596)		(13,199)		(84,697)
Change in net unrealized gains (losses) on investments AFS, net of tax	(99,031)		(11,956)		(134,785)		(32,064)
Grey Castle Life Reinsurance SAC Ltd | Life Funds Withheld Assets
Derivative [Line Items]
Underwriting profit (loss)	0		0		0		0
Net investment income	30,252		31,439		60,650		64,803
Net realized gains (losses) on investments AFS	65,305		29,778		75,663		64,252
Net unrealized gains (losses) on investments, trading	(1,896)		(22,319)		(33,563)		(23,725)
Other-than-temporary impairments	0		0		(1,203)		0
Foreign exchange gains (losses)	(14,794)		14,964		(4,074)		18,259
Other income and expenses	(80)		(19)		(116)		(90)
Net realized and unrealized gains (losses) on life retrocession embedded derivative	(36,120)		(34,596)		(13,199)		(84,697)
Net income (loss)	42,667		19,247		84,158		38,802
Change in net unrealized gains (losses) on investments AFS, net of tax	(99,031)		(11,956)		(134,785)		(32,064)
Change in adjustments related to future policy benefit reserves, net of tax	42,314		3,437		46,478		8,569
Foreign currency translation adjustments, net of tax	14,050		(10,728)		4,149		(15,307)
Total changes to other comprehensive income as a result of GreyCastle Life Retro Arrangements	(42,667)		(19,247)		(84,158)		(38,802)
Comprehensive income (loss)	$ 0		$ 0		$ 0		$ 0

[1] Net fee income and other includes operating expenses of $12.1 million from the Company's loss prevention consulting services business.

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

[3] Net fee income and other includes operating expenses of $24.6 million from the Company's loss prevention consulting services business.

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