Unaudited Interim Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Total Equity
Balance, beginning at Dec. 31, 2013		$ 2,500	$ 804,237	$ 3,542,838	$ 56,646	$ 4,406,221
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend to parent				(338,000)		(338,000)
Contributed (distributed) capital-parent/child asset transfers			1,004			1,004
Comprehensive income (loss):
Net income (loss)	$ 700,418			700,418		700,418
Other comprehensive income (loss), net of tax	76,495				76,495	76,495
Total comprehensive income (loss)						776,913
Balance, ending at Sep. 30, 2014		2,500	805,241	3,905,256	133,141	4,846,138
Balance, beginning at Dec. 31, 2014		2,500	792,153	3,580,641	178,691	4,553,985
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend to parent				(230,000)		(230,000)
Contributed (distributed) capital-parent/child asset transfers			(12,180)			(12,180)
Comprehensive income (loss):
Net income (loss)	164,556			164,556		164,556
Other comprehensive income (loss), net of tax	$ (69,367)				(69,367)	(69,367)
Total comprehensive income (loss)						95,189
Balance, ending at Sep. 30, 2015		$ 2,500	$ 779,973	$ 3,515,197	$ 109,324	$ 4,406,994