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, 2015		$ 2,500	$ 901,422	$ 396,830	$ 46,166	$ 1,346,918
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributed capital	$ 781,125		8,421,955			8,421,955
Assets purchased/transferred from/to affiliates			(72,179)			(72,179)
Return of capital			0			0
Comprehensive income:
Net income (loss)	(1,458,895)			(1,458,895)		(1,458,895)
Other comprehensive income (loss), net of tax	193,745				193,745	193,745
Total comprehensive income (loss)						(1,265,150)
Ending Balance at Jun. 30, 2016		2,500	9,251,198	(1,062,065)	239,911	8,431,544
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]
Contributed capital	0		0			0
Assets purchased/transferred from/to affiliates			0			0
Return of capital			(100,000)			(100,000)
Comprehensive income:
Net income (loss)	(138,225)			(138,225)		(138,225)
Other comprehensive income (loss), net of tax	$ 153,026				153,026	153,026
Total comprehensive income (loss)						14,801
Ending Balance at Jun. 30, 2017		$ 2,500	$ 7,995,436	$ (831,483)	$ (161,922)	$ 7,004,531