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, 2015		$ 2,500	$ 779,973	$ 3,636,410	$ 64,805	$ 4,483,688
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			205,000			205,000
Dividend to parent				(2,593,098)		(2,593,098)
Contributed (distributed) capital-parent/child asset transfers			1,089			1,089
Comprehensive income:
Net income (loss)	$ 405,205			405,205		405,205
Other comprehensive income (loss), net of tax	6,170				6,170	6,170
Total comprehensive income (loss)						411,375
Ending 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
Comprehensive income:
Net income (loss)	11,304			11,304		11,304
Total comprehensive income (loss)						(103,970)
Ending Balance at Mar. 31, 2018				1,515,149		2,743,173
Beginning 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 | ASU 2016-01					(1,539)
Cumulative effect of adoption | ASU 2018-02					$ 30,449