Derivative Instruments - Impact of Life Retro Arrangements (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Underwriting profit (loss)		$ 14,193	$ (1,235,734)	$ 188,261	$ 138,456	$ 115,405	$ 162,192	$ 97,691	$ 172,037	$ (894,824)	$ 547,325	$ 587,409
Net investment income										829,723	827,133	872,370
Exchange (gains) losses										44,620	(9,578)	22,504
Other Operating Income (Expense), Net	[1]									(2,076)	(13,315)	(13,356)
Net realized and unrealized gains (losses) on derivative instruments										(41,732)	2,521	53,123
Net income (loss)										(490,137)	570,145	1,313,339
Cumulative adjustments recorded to value of future policy benefit reserves, amount										27,184	62,295	170,688
Foreign currency translation adjustments										24,072	95,473	(25,315)
Comprehensive income (loss)										173,885	28,930	(797,842)
Life Funds Withheld Assets
Derivative [Line Items]
Net investment income										127,047	154,751	187,489
Net realized gains (losses) on investments sold										115,911	152,589	223,272
Net unrealized gains (losses) on investments, Trading - Life Funds Withheld Assets										(14,805)	109,458	(27,734)
Other-than-temporary impairments (OTTI) on investments										(1,434)	(2,598)	(13,357)
Net realized and unrealized gains (losses) on derivative instruments										(206,015)	(540,090)	(151,691)
Change in net unrealized gains (losses) on investments, net of tax										(51,983)	(4,502)	(421,604)
GreyCastle Life Reinsurance SAC Ltd
Derivative [Line Items]
Comprehensive income (loss)										0	0
GreyCastle Life Reinsurance SAC Ltd | Life Funds Withheld Assets
Derivative [Line Items]
Underwriting profit (loss)	[2]									0	0	605
Net investment income										127,047	154,751	187,489
Net realized gains (losses) on investments sold										115,911	152,589	223,272
Net unrealized gains (losses) on investments, Trading - Life Funds Withheld Assets										(14,805)	109,458	(27,734)
Other-than-temporary impairments (OTTI) on investments										(1,434)	(2,598)	(13,357)
Exchange (gains) losses										30,711	(8,988)	4,788
Other Operating Income (Expense), Net										(108)	(154)	2,280
Net realized and unrealized gains (losses) on derivative instruments										(206,015)	(540,090)	(151,691)
Net income (loss)										51,307	(135,032)	225,652
Change in net unrealized gains (losses) on investments, net of tax										(51,983)	(4,502)	(421,604)
Cumulative adjustments recorded to value of future policy benefit reserves, amount										27,184	62,295	170,688
Foreign currency translation adjustments										(26,508)	77,239	25,869
Change In Other Comprehensive Income Life Retro Arrangements										(51,307)	135,032	(225,047)
Comprehensive income (loss)										$ 0	$ 0	$ 605

[1] Net fee income and other includes operating expenses of $50.7 million from the Company's loss prevention consulting services business.

[2] The underwriting profit of $0.6 million in 2015 relates to premium adjustments relating to the GreyCastle Life Retro Arrangements transaction. Excluding these transactions, the impact to comprehensive income relating to the GreyCastle Life Retro Arrangements was nil for the years ended December 31, 2017 and 2016.