Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Other Comprehensive Income [Roll Forward]
Beginning balance				$ 2,012,321	$ 1,882,266
Other comprehensive income (loss) before reclassifications		$ 106,081	$ 196,757	167,706	436,454
Amounts reclassified from AOCI		204	(17,161)	2,558	(30,305)
Other comprehensive income (loss)	[1]	106,285	179,596	170,264	406,149
Ending balance		2,207,290	2,357,469	2,207,290	2,357,469
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Beginning balance		398,293	616,836	311,748	339,520
Other comprehensive income (loss) before reclassifications		154,047	255,535	238,696	545,831
Amounts reclassified from AOCI		(1,564)	(17,337)	332	(30,317)
Other comprehensive income (loss)		152,483	238,198	239,028	515,514
Ending balance		550,776	855,034	550,776	855,034
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Beginning balance		61,618	42,625	67,076	45,284
Other comprehensive income (loss) before reclassifications		(11,859)	3,967	(16,380)	2,924
Amounts reclassified from AOCI		373	(1,276)	(564)	(2,892)
Other comprehensive income (loss)		(11,486)	2,691	(16,944)	32
Ending balance		50,132	45,316	50,132	45,316
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Beginning balance		(77,149)	(115,341)	(58,646)	(65,785)
Other comprehensive income (loss) before reclassifications		(36,107)	(62,745)	(54,610)	(112,301)
Amounts reclassified from AOCI		0	0	0	0
Other comprehensive income (loss)		(36,107)	(62,745)	(54,610)	(112,301)
Ending balance		(113,256)	(178,086)	(113,256)	(178,086)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Beginning balance		(82,908)	(84,129)	(84,303)	(85,581)
Other comprehensive income (loss) before reclassifications		0	0	0	0
Amounts reclassified from AOCI		1,395	1,452	2,790	2,904
Other comprehensive income (loss)		1,395	1,452	2,790	2,904
Ending balance		(81,513)	(82,677)	(81,513)	(82,677)
AOCI Attributable to Parent
Other Comprehensive Income [Roll Forward]
Beginning balance		299,854	459,991	235,875	233,438
Other comprehensive income (loss)				170,264	406,149
Ending balance		$ 406,139	$ 639,587	$ 406,139	$ 639,587

[1] Other comprehensive income includes the non-credit component of impaired gains (losses), net, on fixed maturities available-for-sale in the amounts of $(520) and $(2,472) for the three months ended June 30, 2017, and 2016, respectively and $(1,609) and $(4,367) for the six months ended June 30, 2017, and 2016, respectively.