Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		$ 409,371	$ 645,085	$ 603,018	$ 345,754
Other comprehensive income (loss) before reclassifications		4,448	(62,601)	(169,954)	255,420
Amounts reclassified from AOCI		(3,202)	(1,578)	(22,447)	(20,268)
Other comprehensive income (loss)	[1]	1,246	(64,179)	(192,401)	235,152
Balance at the end of the period		410,617	580,906	410,617	580,906
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		560,527	790,332	784,183	434,023
Other comprehensive income (loss) before reclassifications		(3,619)	(65,520)	(207,043)	306,499
Amounts reclassified from AOCI		(3,859)	(2,664)	(24,091)	(18,374)
Other comprehensive income (loss)		(7,478)	(68,184)	(231,134)	288,125
Balance at the end of the period		553,049	722,148	553,049	722,148
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		30,653	25,068	33,141	25,517
Other comprehensive income (loss) before reclassifications		9,623	4,815	9,962	7,346
Amounts reclassified from AOCI		(1,364)	(1,716)	(4,191)	(4,696)
Other comprehensive income (loss)		8,259	3,099	5,771	2,650
Balance at the end of the period		38,912	28,167	38,912	28,167
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		(79,296)	(126,529)	(108,194)	(70,000)
Other comprehensive income (loss) before reclassifications		(1,549)	263	27,349	(56,266)
Amounts reclassified from AOCI		0	0	0	0
Other comprehensive income (loss)		(1,549)	263	27,349	(56,266)
Balance at the end of the period		(80,845)	(126,266)	(80,845)	(126,266)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		(102,513)	(43,786)	(106,112)	(43,786)
Other comprehensive income (loss) before reclassifications		(7)	(2,159)	(222)	(2,159)
Amounts reclassified from AOCI		2,021	2,802	5,835	2,802
Other comprehensive income (loss)		2,014	643	5,613	643
Balance at the end of the period		$ (100,499)	$ (43,143)	$ (100,499)	$ (43,143)

[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 $(1,979) and $(996) for the three months ended September 30, 2015, and 2014, respectively, and $(3,208) and $(3,812) for the nine months ended September 30, 2015, and 2014, respectively.