Statements of Equity - USD ($) $ in Thousands	Total	Common   Stock	Additional   Paid-in Capital	Retained Earnings/  (Accumulated Deficit)	Retained Earnings/  (Accumulated Deficit) Cumulative Effect, Period of Adoption, Adjustment		Retained Earnings/  (Accumulated Deficit) ASU 2016-01	Retained Earnings/  (Accumulated Deficit) ASU 2018-02	Accumulated Other Comprehensive   Income (loss)	Accumulated Other Comprehensive   Income (loss) ASU 2016-01	Accumulated Other Comprehensive   Income (loss) ASU 2018-02	Total Equity	Total Equity   Cumulative Effect, Period of Adoption, Adjustment		Total Equity   ASU 2016-01	Total Equity   ASU 2018-02
Beginning Balance at Dec. 31, 2017		$ 2,500	$ 7,145,436	$ (776,762)			$ 337	$ 36,714	$ (90,124)	$ (3)	$ (36,714)	$ 6,281,050			$ 334	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201601Member
Return of capital			(1,025,000)									(1,025,000)
Comprehensive income (loss):
Net income (loss)	$ 1,682,716			1,682,716								1,682,716
Other comprehensive income (loss), net of tax	$ (197,532)								(197,532)			(197,532)
Total comprehensive income (loss)												1,485,184
Ending Balance at Dec. 31, 2018		2,500	6,120,436	943,005	$ (371)	[1]			(324,373)			6,741,568	$ (371)	[1]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201708Member
Return of capital			(977,500)									(977,500)
Comprehensive income (loss):
Net income (loss)	$ (989,327)			(989,327)								(989,327)
Other comprehensive income (loss), net of tax	$ 752,985								752,985			752,985
Total comprehensive income (loss)												(236,342)
Ending Balance at Dec. 31, 2019		2,500	5,142,936	(46,693)	$ (1,392)	[2]			428,612			5,527,355	$ (1,392)	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Return of capital			(760,000)									(760,000)
Comprehensive income (loss):
Net income (loss)	$ (3,169,265)			(3,169,265)								(3,169,265)
Other comprehensive income (loss), net of tax	$ 1,104,447								1,104,447			1,104,447
Total comprehensive income (loss)												(2,064,818)
Ending Balance at Dec. 31, 2020		$ 2,500	$ 4,382,936	$ (3,217,350)					$ 1,533,059			$ 2,701,145

[1] Includes the impact from the adoption of ASU 2017-08 and 2017-12.

[2] Includes the impact from the adoption of ASU 2016-13. See Note 2.