Other Comprehensive Income - OCI - Composition of Other comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Before-tax amount
Unrealized holding gains (losses), net, arising on fixed maturities, available-for-sale		$ 39,983	$ 55,849	$ 407,208	$ 895,589
Unrealized holding gains (losses), net, arising on cash flow hedges		(23,111)	(7,805)	(48,311)	(3,307)
Reclassification adjustment for (gains) losses, net, realized in net income		(2,885)	(10,284)	(3,242)	(61,375)
Net unrealized gains (losses) related to investments		13,987	37,760	355,655	830,907
Future policy benefits, DAC and VOBA adjustments		(1,977)	(18,214)	(85,992)	(190,985)
Net unrealized gains (losses)		12,010	19,546	269,663	639,922
Employee benefit plan adjustment		10,744	2,494	15,036	6,962
Other comprehensive income before income taxes		22,754	22,040	284,699	646,884
Tax (expense) benefit
Unrealized holding gains (losses), net, arising on fixed maturities, available-for-sale		(13,994)	(19,547)	(142,523)	(313,456)
Unrealized holding gains (losses), net, arising on cash flow hedges		8,089	2,731	16,909	1,157
Reclassification adjustment for (gains) losses, net, realized in net income		1,010	3,599	1,135	21,481
Net unrealized gains (losses) related to investments		(4,895)	(13,217)	(124,479)	(290,818)
Future policy benefits, DAC and VOBA adjustments		692	6,375	30,097	66,845
Net unrealized gains (losses)		(4,203)	(6,842)	(94,382)	(223,973)
Employee benefit plan adjustment		(3,760)	(873)	(5,262)	(2,437)
Other comprehensive income (loss)		(7,963)	(7,715)	(99,644)	(226,410)
Net-of-tax amount
Unrealized holding gains (losses), net, arising on fixed maturities, available-for-sale		25,989	36,302	264,685	582,133
Unrealized holding gains (losses), net, arising on cash flow hedges		(15,022)	(5,074)	(31,402)	(2,150)
Reclassification adjustment for (gains) losses, net, realized in net income		(1,875)	(6,685)	(2,107)	(39,894)
Net unrealized gains (losses) related to investments		9,092	24,543	231,176	540,089
Future policy benefits, DAC and VOBA adjustments		(1,285)	(11,839)	(55,895)	(124,140)
Net unrealized gains (losses)		7,807	12,704	175,281	415,949
Employee benefit plan adjustment		6,984	1,621	9,774	4,525
Other comprehensive income (loss)	[1]	$ 14,791	$ 14,325	$ 185,055	$ 420,474

[1] Other comprehensive income includes the non-credit component of impaired gains (losses), net, on fixed maturities available-for-sale in the amounts of $(2,258) and $(959) for the three months ended September 30, 2017, and 2016, respectively and $(3,867) and $(5,326) for the nine months ended September 30, 2017, and 2016, respectively.