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, 2017		$ 2,500	$ 7,145,436	$ (776,762)	$ (90,124)	$ 6,281,050
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital			(300,000)			(300,000)
Comprehensive income:
Net income (loss)				635,679		635,679
Other comprehensive income (loss), net of tax					(139,866)	(139,866)
Total comprehensive income (loss)						495,813
Ending Balance at Mar. 31, 2018		2,500	6,845,436	(104,032)	(266,707)	6,477,197
Beginning Balance at Dec. 31, 2017		2,500	7,145,436	(776,762)	(90,124)	6,281,050
Comprehensive income:
Net income (loss)	$ 838,225
Other comprehensive income (loss), net of tax	(186,001)
Ending Balance at Jun. 30, 2018		2,500	6,595,436	98,514	(312,842)	6,383,608
Beginning Balance at Mar. 31, 2018		2,500	6,845,436	(104,032)	(266,707)	6,477,197
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital			(250,000)			(250,000)
Comprehensive income:
Net income (loss)	202,546			202,546		202,546
Other comprehensive income (loss), net of tax	(46,135)				(46,135)	(46,135)
Total comprehensive income (loss)						156,411
Ending Balance at Jun. 30, 2018		2,500	6,595,436	98,514	(312,842)	6,383,608
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption | ASU 2016-01					(3)
Cumulative effect of adoption | ASU 2018-02					(36,714)
Cumulative effect of adoption	42
Beginning Balance at Dec. 31, 2018		2,500	6,120,436	943,005	(324,373)	6,741,568
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital			(245,000)			(245,000)
Comprehensive income:
Net income (loss)				(900,024)		(900,024)
Other comprehensive income (loss), net of tax					224,777	224,777
Total comprehensive income (loss)						(675,247)
Ending Balance at Mar. 31, 2019		2,500	5,875,436	42,610	(99,596)	5,820,950
Beginning Balance at Dec. 31, 2018		2,500	6,120,436	943,005	(324,373)	6,741,568
Comprehensive income:
Net income (loss)	(1,068,794)
Other comprehensive income (loss), net of tax	559,172
Ending Balance at Jun. 30, 2019		2,500	5,628,936	(126,160)	234,799	5,740,075
Beginning Balance at Mar. 31, 2019		2,500	5,875,436	42,610	(99,596)	5,820,950
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Return of capital			(246,500)			(246,500)
Comprehensive income:
Net income (loss)	(168,770)			(168,770)		(168,770)
Other comprehensive income (loss), net of tax	$ 334,395				334,395	334,395
Total comprehensive income (loss)						165,625
Ending Balance at Jun. 30, 2019		$ 2,500	$ 5,628,936	$ (126,160)	$ 234,799	$ 5,740,075