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, 2014		$ 2,500	$ 792,153	$ 3,557,144	$ 178,691	$ 4,530,488
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			0			0
Dividend to parent				(430,000)		(430,000)
Contributed (distributed) capital-parent/child asset transfers			(12,180)			(12,180)
Comprehensive income:
Net income (loss)	$ 508,003			508,003		508,003
Other comprehensive income (loss), net of tax	(113,886)				(113,886)	(113,886)
Total comprehensive income (loss)						394,117
Ending Balance at Dec. 31, 2015		2,500	779,973	3,635,147	64,805	4,482,425
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)	395,217			395,217		395,217
Other comprehensive income (loss), net of tax	6,170				6,170	6,170
Total comprehensive income (loss)						401,387
Ending Balance at Dec. 31, 2016		2,500	986,062	1,437,266	70,975	2,496,803
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)	324,432			324,432		324,432
Other comprehensive income (loss), net of tax	$ 94,371				94,371	94,371
Total comprehensive income (loss)						418,803
Ending Balance at Dec. 31, 2017		$ 2,500	$ 1,141,092	$ 1,511,698	$ 165,346	$ 2,820,636