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, 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				(230,000)		(230,000)
Contributed (distributed) capital-parent/child asset transfers			(12,180)			(12,180)
Comprehensive income (loss):
Net income (loss)	159,810			159,810		159,810
Other comprehensive income (loss), net of tax	(69,367)				(69,367)	(69,367)
Total comprehensive income (loss)						90,443
Balance, ending at Sep. 30, 2015		2,500	779,973	3,486,954	109,324	4,378,751
Balance, beginning 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
Balance, ending 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			205,000
Dividend to parent				(2,593,519)		(2,593,519)
Contributed (distributed) capital-parent/child asset transfers			1,090			1,090
Comprehensive income (loss):
Net income (loss)	480,682			480,682		480,682
Other comprehensive income (loss), net of tax	$ 184,982				184,982	184,982
Total comprehensive income (loss)						665,664
Balance, ending at Sep. 30, 2016		$ 2,500	$ 986,063	$ 1,522,310	$ 249,787	$ 2,760,660