Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Other Comprehensive Income [Roll Forward]
Beginning balance		$ 639,587	$ 409,371	$ 233,438	$ 603,018
Other comprehensive income (loss) before reclassifications		19,389	4,448	455,843	(169,954)
Amounts reclassified from AOCI		(5,064)	(3,202)	(35,369)	(22,447)
Other comprehensive income (loss)	[1]	14,325	1,246	420,474	(192,401)
Ending balance		653,912	410,617	653,912	410,617
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Beginning balance		855,034	560,527	339,520	784,183
Other comprehensive income (loss) before reclassifications		36,302	(3,619)	582,133	(207,043)
Amounts reclassified from AOCI		(5,503)	(3,859)	(35,820)	(24,091)
Other comprehensive income (loss)		30,799	(7,478)	546,313	(231,134)
Ending balance		885,833	553,049	885,833	553,049
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Beginning balance		45,316	30,653	45,284	33,141
Other comprehensive income (loss) before reclassifications		(5,074)	9,623	(2,150)	9,962
Amounts reclassified from AOCI		(1,182)	(1,364)	(4,074)	(4,191)
Other comprehensive income (loss)		(6,256)	8,259	(6,224)	5,771
Ending balance		39,060	38,912	39,060	38,912
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Beginning balance		(178,086)	(79,296)	(65,785)	(108,194)
Other comprehensive income (loss) before reclassifications		(11,839)	(1,549)	(124,140)	27,349
Amounts reclassified from AOCI		0	0	0	0
Other comprehensive income (loss)		(11,839)	(1,549)	(124,140)	27,349
Ending balance		(189,925)	(80,845)	(189,925)	(80,845)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Beginning balance		(82,677)	(102,513)	(85,581)	(106,112)
Other comprehensive income (loss) before reclassifications		0	(7)	0	(222)
Amounts reclassified from AOCI		1,621	2,021	4,525	5,835
Other comprehensive income (loss)		1,621	2,014	4,525	5,613
Ending balance		$ (81,056)	$ (100,499)	$ (81,056)	$ (100,499)

[1] Other comprehensive income (loss) includes the non-credit component of impaired gains (losses), net, on fixed maturities available-for-sale in the amounts of $(959) and $(1,979) for the three months ended September 30, 2016, and 2015, respectively, and $(5,326) and $(3,208) for the nine months ended September 30, 2016, and 2015, respectively.