Other Comprehensive Income (Details 2) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Before-tax amount
Unrealized holding gains (losses), net, arising on fixed maturities, available-for-sale		$ 236,996	$ 393,130	$ 367,225	$ 839,740
Unrealized holding gains (losses), net, arising on cash flow hedges		(18,245)	6,102	(25,200)	4,498
Reclassification adjustment for (gains) losses, net, realized in net income		(1,832)	(28,635)	(357)	(51,091)
Net unrealized gains (losses) related to investments		216,919	370,597	341,668	793,147
Future policy benefits, DAC and VOBA adjustments		(55,549)	(96,531)	(84,015)	(172,771)
Net unrealized gains (losses)		161,370	274,066	257,653	620,376
Employee benefit plan adjustment		2,146	2,234	4,292	4,468
Other comprehensive income before income taxes		163,516	276,300	261,945	624,844
Tax (expense) benefit
Unrealized holding gains (losses), net, arising on fixed maturities, available-for-sale		(82,949)	(137,595)	(128,529)	(293,909)
Unrealized holding gains (losses), net, arising on cash flow hedges		6,386	(2,135)	8,820	(1,574)
Reclassification adjustment for (gains) losses, net, realized in net income		641	10,022	125	17,882
Net unrealized gains (losses) related to investments		(75,922)	(129,708)	(119,584)	(277,601)
Future policy benefits, DAC and VOBA adjustments		19,442	33,786	29,405	60,470
Net unrealized gains (losses)		(56,480)	(95,922)	(90,179)	(217,131)
Employee benefit plan adjustment		(751)	(782)	(1,502)	(1,564)
Other comprehensive income (loss)		(57,231)	(96,704)	(91,681)	(218,695)
Net-of-tax amount
Unrealized holding gains (losses), net, arising on fixed maturities, available-for-sale		154,047	255,535	238,696	545,831
Unrealized holding gains (losses), net, arising on cash flow hedges		(11,859)	3,967	(16,380)	2,924
Reclassification adjustment for (gains) losses, net, realized in net income		(1,191)	(18,613)	(232)	(33,209)
Net unrealized gains (losses) related to investments		140,997	240,889	222,084	515,546
Future policy benefits, DAC and VOBA adjustments		(36,107)	(62,745)	(54,610)	(112,301)
Net unrealized gains (losses)		104,890	178,144	167,474	403,245
Employee benefit plan adjustment		1,395	1,452	2,790	2,904
Other comprehensive income (loss)	[1]	$ 106,285	$ 179,596	$ 170,264	$ 406,149

[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.