Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income		$ 369,116	$ 231,092	$ 190,455
Components of other comprehensive income (loss)
Unrealized holding gains (losses), net, arising on available-for-sale fixed maturity investments		377,605	20,295	(643,880)
Unrealized holding (losses) gains, net, arising on cash flow hedges		(73,726)	44,776	31,061
Reclassification adjustment for (gains), net, realized in net income		34,208	74,271	52,597
Net unrealized gains (losses) related to investments		269,671	(9,200)	(665,416)
Future policy benefits, DAC and VOBA adjustments		(88,648)	10,983	65,245
Employee benefit plan adjustment		14,265	1,966	31,586
Other, net		(74,383)	12,949	96,831
Other comprehensive income (loss) before income taxes		195,288	3,749	(568,585)
Income tax expense (benefit) related to items of other comprehensive income		68,351	1,312	(199,005)
Other comprehensive income (loss)	[1]	126,937	2,437	(369,580)
Total comprehensive income (loss)		$ 496,053	$ 233,529	$ (179,125)

[1] Other comprehensive income (loss) includes the non-credit component of impaired losses on fixed maturities available-for-sale, net of future policy benefits, DAC and VOBA adjustments and income taxes, in the amounts of $(1,362), $(6,520), and $(6,596) for the years ended December 31, 2017, 2016, and 2015, respectively.