Accumulated Other Comprehensive Income - Details about AOCI components (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized and unrealized gains (losses) on derivative instruments		$ (41,732)	$ 2,521	$ 53,123
Operating Expenses		(1,757,059)	(2,063,362)	(1,966,939)
Exchange gains (losses) before tax		(44,620)	9,578	(22,504)
Amounts reclassified from AOCI		(291,429)	(344,393)	(326,979)
Provision for income tax		(59,070)	(42,129)	19,161
Unrealized Gains (Losses) on Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI	[1]	(298,852)	(353,910)	(346,911)
OTTI Losses Recognized in AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI		7,488	6,599	18,987
Foreign Currency Translation Adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI		0	0	0
Underfunded Pension Liability
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI		127	3,100	1,145
Cash Flow Hedge
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI		(192)	(182)	(200)
Life Funds Withheld Assets
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on investments sold		115,911	152,589	223,272
OTTI on investments		(1,434)	(2,598)	(13,357)
Net realized and unrealized gains (losses) on derivative instruments		(206,015)	(540,090)	$ (151,691)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI		(291,429)	(344,393)
Provision for income tax		3,053	8,950
Total amount reclassified from AOCI, net of tax		(288,376)	(335,443)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on investments sold		(258,590)	(350,875)
OTTI on investments		14,422	81,595
Other Comprehensive Income Loss Reclassification Adjustment From Aoci For Investments Transferred From Available for sale To Equity Method		0	33,530
Amounts reclassified from AOCI		(298,852)	(353,910)
Provision for income tax		3,161	10,084
Total amount reclassified from AOCI, net of tax		(295,691)	(343,826)
Reclassification out of Accumulated Other Comprehensive Income [Member] | OTTI Losses Recognized in AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) on investments sold		7,129	7,038
OTTI on investments		359	(439)
Amounts reclassified from AOCI		7,488	6,599
Provision for income tax		(63)	(49)
Total amount reclassified from AOCI, net of tax		7,425	6,550
Reclassification out of Accumulated Other Comprehensive Income [Member] | Underfunded Pension Liability
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating Expenses		127	3,100
Provision for income tax		(45)	(1,085)
Total amount reclassified from AOCI, net of tax		82	2,015
Reclassification out of Accumulated Other Comprehensive Income [Member] | Cash Flow Hedge
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI		(192)	(182)
Provision for income tax		0	0
Total amount reclassified from AOCI, net of tax		(192)	(182)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Life Funds Withheld Assets | Unrealized Gains (Losses) on Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net adjustments related to future policy benefit reserves, net of tax		$ (54,684)	$ (51,100)

[1] For certain annuity contracts that are subject to the GreyCastle Life Retro Arrangements, policy benefit reserves were historically increased for the impact of changes in unrealized gains on investments supporting such contracts as if the gains had been realized, with a corresponding entry to other comprehensive income ("Shadow Adjustments"). Upon completion of the GreyCastle Life Retro Arrangements, no further Shadow Adjustments were recorded. As of December 31, 2016, the cumulative impact of the Shadow Adjustments was $212.1 million. During the year ended December 31, 2017, net movement of $(27.2) million were recorded, resulting in a total cumulative net impact of Shadow Adjustments on future policy benefit reserves of $184.9 million at December 31, 2017.