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		635,679
Other comprehensive income (loss), net of tax		(139,866)				(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
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
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)		(900,024)
Other comprehensive income (loss), net of tax		$ 224,777				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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption	[1]				$ (371)	$ 0	$ (371)

[1]

(1) Includes the impact from the adoption of ASUs 2017-08 and 2017-12. See Note 2.