Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		$ 691,137	$ 497,020	$ 603,018	$ 345,754
Other comprehensive income (loss) before reclassifications		(280,398)	152,313	(174,402)	318,021
Amounts reclassified from AOCI		(1,368)	(4,248)	(19,245)	(18,690)
Other comprehensive income (loss)	[1]	(281,766)	148,065	(193,647)	299,331
Balance at the end of the period		409,371	645,085	409,371	645,085
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		883,761	622,302	784,183	434,023
Other comprehensive income (loss) before reclassifications		(321,318)	169,675	(203,424)	372,019
Amounts reclassified from AOCI		(1,916)	(1,645)	(20,232)	(15,710)
Other comprehensive income (loss)		(323,234)	168,030	(223,656)	356,309
Balance at the end of the period		560,527	790,332	560,527	790,332
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		42,824	25,715	33,141	25,517
Other comprehensive income (loss) before reclassifications		(10,812)	1,956	339	2,531
Amounts reclassified from AOCI		(1,359)	(2,603)	(2,827)	(2,980)
Other comprehensive income (loss)		(12,171)	(647)	(2,488)	(449)
Balance at the end of the period		30,653	25,068	30,653	25,068
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		(131,028)	(107,211)	(108,194)	(70,000)
Other comprehensive income (loss) before reclassifications		51,732	(19,318)	28,898	(56,529)
Amounts reclassified from AOCI		0	0	0	0
Other comprehensive income (loss)		51,732	(19,318)	28,898	(56,529)
Balance at the end of the period		(79,296)	(126,529)	(79,296)	(126,529)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Balance at the beginning of the period		(104,420)	(43,786)	(106,112)	(43,786)
Other comprehensive income (loss) before reclassifications		0	0	(215)	0
Amounts reclassified from AOCI		1,907	0	3,814	0
Other comprehensive income (loss)		1,907	0	3,599	0
Balance at the end of the period		$ (102,513)	$ (43,786)	$ (102,513)	$ (43,786)

[1] Other comprehensive (loss) income includes the non-credit component of impaired gains (losses), net, on fixed maturities available-for-sale in the amounts of $1,314 and $(4,588) for the three months ended June 30, 2015, and 2014, respectively, and $(1,229) and $(2,816) for the six months ended June 30, 2015, and 2014, respectively.