Consolidated Statements of Stockholder's Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income	Total Equity
Beginning Balance at Dec. 31, 2016		$ 2,500	$ 986,062	$ 1,448,517	$ 70,975	$ 2,508,054
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			153,500			153,500
Dividend to parent				(250,000)		(250,000)
Contributed (distributed) capital-parent/child asset transfers			1,530			1,530
Comprehensive income:
Net income (loss)	$ 327,793			327,793		327,793
Other comprehensive income (loss), net of tax	94,371				94,371	94,371
Total comprehensive income (loss)						422,164
Ending Balance at Dec. 31, 2017		2,500	1,141,092	1,526,310	165,346	2,835,248
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			5,500			5,500
Comprehensive income:
Net income (loss)	108,638			108,638		108,638
Other comprehensive income (loss), net of tax	(222,552)				(222,552)	(222,552)
Total comprehensive income (loss)						(113,914)
Ending Balance at Dec. 31, 2018		2,500	1,146,592	1,612,435	(28,296)	2,733,231
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption	(27)
Cumulative effect of adoption | ASU 2016-01					(1,539)
Contributed capital			5,900			5,900
Dividend to parent				(250,000)		(250,000)
Contributed (distributed) capital-parent/child asset transfers			1,140	0		1,140
Comprehensive income:
Net income (loss)	216,999			216,999		216,999
Other comprehensive income (loss), net of tax	$ 309,738				309,738	309,738
Total comprehensive income (loss)						526,737
Ending Balance at Dec. 31, 2019		$ 2,500	$ 1,153,632	$ 1,577,453	$ 281,442	$ 3,015,027