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	$ 3,580,641	$ 178,691	$ 4,553,985
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)	298,659			298,659		298,659
Other comprehensive income (loss), net of tax	(83,591)				(83,591)	(83,591)
Total comprehensive income (loss)						215,068
Balance, ending at Jun. 30, 2015		2,500	779,973	3,649,300	95,100	4,526,873
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]
Contributed capital						0
Balance, ending at Dec. 31, 2015		2,500	779,973	3,663,539	64,805	4,510,817
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)	368,675			368,675		368,675
Other comprehensive income (loss), net of tax	$ 176,211				176,211	176,211
Total comprehensive income (loss)						544,886
Balance, ending at Jun. 30, 2016		$ 2,500	$ 986,063	$ 1,438,695	$ 241,016	$ 2,668,274