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, 2013		$ 2,500	$ 804,237	$ 3,525,476	$ 56,646	$ 4,388,859
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital			0
Dividend to parent				(748,000)		(748,000)
Contributed (distributed) capital-parent/child asset transfers			(12,084)			(12,084)
Comprehensive income:
Net income (loss)	$ 779,668			779,668		779,668
Other comprehensive income (loss), net of tax	122,045				122,045	122,045
Total comprehensive income (loss)						901,713
Ending 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
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
Comprehensive income:
Net income (loss)	(328,199)			(328,199)		(328,199)
Total comprehensive income (loss)						(212,872)
Ending Balance at Mar. 31, 2016				3,306,948		4,274,493
Beginning Balance at Dec. 31, 2015		2,500	779,973	3,635,147	64,805	4,482,425
Comprehensive income:
Net income (loss)	395,785			395,785		395,785
Total comprehensive income (loss)						571,996
Ending Balance at Jun. 30, 2016				1,437,413		2,666,992
Beginning 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
Beginning Balance at Mar. 31, 2016				3,306,948		4,274,493
Comprehensive income:
Net income (loss)	$ 723,984
Ending Balance at Jun. 30, 2016				$ 1,437,413		$ 2,666,992