Statements of Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income	Total Equity
Balance at Dec. 31, 2012		$ 2,500	$ 893,336	$ 200,754	$ 147,287	$ 1,243,877
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			8,086			8,086
Dividend to parent				(284,000)		(284,000)
Comprehensive income:
Net income	$ 848,092			848,092		848,092
Other comprehensive income (loss), net of taxes	(76,420)				(76,420)	(76,420)
Total comprehensive income						771,672
Balance at Dec. 31, 2013		2,500	901,422	764,846	70,867	1,739,635
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend to parent				(342,000)		(342,000)
Comprehensive income:
Net income	250,767			250,767		250,767
Other comprehensive income (loss), net of taxes	13,755				13,755	13,755
Total comprehensive income						264,522
Balance at Dec. 31, 2014		2,500	901,422	673,613	84,622	1,662,157
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend to parent				(450,000)		(450,000)
Comprehensive income:
Net income	173,217			173,217		173,217
Other comprehensive income (loss), net of taxes	$ (38,456)				(38,456)	(38,456)
Total comprehensive income						134,761
Balance at Dec. 31, 2015		$ 2,500	$ 901,422	$ 396,830	$ 46,166	$ 1,346,918