Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Other Comprehensive Income [Roll Forward]
Beginning balance		$ 2,012,321	$ 1,882,266
Other comprehensive income (loss) before reclassifications		61,625	239,697
Amounts reclassified from AOCI		2,354	(13,144)
Other comprehensive income (loss)	[1]	63,979	226,553
Ending balance		2,034,550	2,134,719
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Beginning balance		311,748	339,520
Other comprehensive income (loss) before reclassifications		84,649	290,296
Amounts reclassified from AOCI		1,896	(12,980)
Other comprehensive income (loss)		86,545	277,316
Ending balance		398,293	616,836
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Beginning balance		67,076	45,284
Other comprehensive income (loss) before reclassifications		(4,521)	(1,043)
Amounts reclassified from AOCI		(937)	(1,616)
Other comprehensive income (loss)		(5,458)	(2,659)
Ending balance		61,618	42,625
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Beginning balance		(58,646)	(65,785)
Other comprehensive income (loss) before reclassifications		(18,503)	(49,556)
Amounts reclassified from AOCI		0	0
Other comprehensive income (loss)		(18,503)	(49,556)
Ending balance		(77,149)	(115,341)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Beginning balance		(84,303)	(85,581)
Other comprehensive income (loss) before reclassifications		0	0
Amounts reclassified from AOCI		1,395	1,452
Other comprehensive income (loss)		1,395	1,452
Ending balance		(82,908)	(84,129)
AOCI Attributable to Parent
Other Comprehensive Income [Roll Forward]
Beginning balance		235,875	233,438
Other comprehensive income (loss)		63,979	226,553
Ending balance		$ 299,854	$ 459,991

[1] Other comprehensive income includes the non-credit component of impaired gains (losses), net, on fixed maturities available-for-sale in the amounts of $(1,089) and $(1,895) for the three months ended March 31, 2017, and 2016, respectively