Unaudited Interim Statements of Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income (Loss)	Total Equity
Beginning Balance at Dec. 31, 2016		$ 2,500	$ 8,095,436	$ (693,258)	$ (314,948)	$ 7,089,730
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital			(300,000)			(300,000)
Comprehensive income:
Net income (loss)	$ 800,701			800,701		800,701
Other comprehensive income (loss), net of tax	147,871				147,871	147,871
Total comprehensive income (loss)						948,572
Ending Balance at Sep. 30, 2017		2,500	7,795,436	107,443	(167,077)	7,738,302
Beginning Balance at Dec. 31, 2017		2,500	7,145,436	(776,762)	(90,124)	6,281,050
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital			(800,000)			(800,000)
Comprehensive income:
Net income (loss)	1,250,461			1,250,461		1,250,461
Other comprehensive income (loss), net of tax	$ (303,853)				(303,853)	(303,853)
Total comprehensive income (loss)						946,608
Ending Balance at Sep. 30, 2018		$ 2,500	$ 6,345,436	510,750	(430,694)	6,427,992
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption | ASU 2016-01				337	(3)	334
Cumulative effect of adoption | ASU 2018-02				$ 36,714	$ (36,714)	$ 0