Condensed Consolidated Statements of Comprehensive (Loss) Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 71,382	$ 46,581	$ 218,668	$ 131,237
Components of other comprehensive (loss) income
Unrealized holding (losses) gains, net, arising on available-for-sale fixed maturity investments	(148,340)	39,983	(961,854)	407,208
Unrealized holding gains (losses), net, arising on cash flow hedges	4,063	(23,111)	56,506	(48,311)
Reclassification adjustment for (gains) losses, net, realized in net income	(6,937)	(2,885)	(24,244)	(3,242)
Net unrealized (losses) gains related to investments	(151,214)	13,987	(929,592)	355,655
Future policy benefits, DAC and VOBA adjustments	35,053	(1,977)	189,415	(85,992)
Employee benefit plan adjustment	646	10,744	1,805	15,036
Other comprehensive (loss) income before income taxes	(115,515)	22,754	(738,372)	284,699
Income tax (benefit) expense related to items of other comprehensive income	(24,259)	7,963	(155,059)	99,644
Other comprehensive income (loss)	(91,256)	14,791	(583,313)	185,055
Total comprehensive (loss) income	$ (19,874)	$ 61,372	$ (364,645)	$ 316,292