Other Comprehensive Income - OCI - Changes in Accumulated 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
Other Comprehensive Income [Roll Forward]
Beginning balance				$ 2,012,321	$ 1,882,266
Other comprehensive income (loss) before reclassifications		$ 15,241	$ 19,389	182,947	455,843
Amounts reclassified from AOCI		(450)	(5,064)	2,108	(35,369)
Other comprehensive income (loss)	[1]	14,791	14,325	185,055	420,474
Ending balance		2,260,897	2,391,593	2,260,897	2,391,593
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Beginning balance		550,776	855,034	311,748	339,520
Other comprehensive income (loss) before reclassifications		25,989	36,302	264,685	582,133
Amounts reclassified from AOCI		(1,630)	(5,503)	(1,298)	(35,820)
Other comprehensive income (loss)		24,359	30,799	263,387	546,313
Ending balance		575,135	885,833	575,135	885,833
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Beginning balance		50,132	45,316	67,076	45,284
Other comprehensive income (loss) before reclassifications		(15,022)	(5,074)	(31,402)	(2,150)
Amounts reclassified from AOCI		(245)	(1,182)	(809)	(4,074)
Other comprehensive income (loss)		(15,267)	(6,256)	(32,211)	(6,224)
Ending balance		34,865	39,060	34,865	39,060
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Beginning balance		(113,256)	(178,086)	(58,646)	(65,785)
Other comprehensive income (loss) before reclassifications		(1,285)	(11,839)	(55,895)	(124,140)
Amounts reclassified from AOCI		0	0	0	0
Other comprehensive income (loss)		(1,285)	(11,839)	(55,895)	(124,140)
Ending balance		(114,541)	(189,925)	(114,541)	(189,925)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Beginning balance		(81,513)	(82,677)	(84,303)	(85,581)
Other comprehensive income (loss) before reclassifications		5,559	0	5,559	0
Amounts reclassified from AOCI		1,425	1,621	4,215	4,525
Other comprehensive income (loss)		6,984	1,621	9,774	4,525
Ending balance		(74,529)	(81,056)	(74,529)	(81,056)
AOCI Attributable to Parent
Other Comprehensive Income [Roll Forward]
Beginning balance		406,139	639,587	235,875	233,438
Other comprehensive income (loss)				185,055	420,474
Ending balance		$ 420,930	$ 653,912	$ 420,930	$ 653,912

[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.