Condensed Consolidated Statements of Changes in Equity (Deficit) - USD ($) shares in Millions, $ in Millions	Total	Senior Preferred Stock	Preferred Stock	Common Stock	Accumulated Deficit	Accumulated Other Comprehensive Income	Treasury Stock	Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Senior Preferred Stock	Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Preferred Stock	Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Common Stock	Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Accumulated Deficit	Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Accumulated Other Comprehensive Income	Cumulative Effect, Period of Adoption, Adjusted Balance [Member] Treasury Stock	Cumulative Effect, Period of Adoption, Adjustment [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Accumulated Deficit
Balance (shares) at Dec. 31, 2019		1	556	1,158					1	556	1,158
Balance at Dec. 31, 2019	$ 14,608	$ 120,836	$ 19,130	$ 687	$ (118,776)	$ 131	$ (7,400)	$ 13,469	$ 120,836	$ 19,130	$ 687	$ (119,915)	$ 131	$ (7,400)
Comprehensive income:
Net income	461				461
Other comprehensive income (loss), net of tax effect:
Changes in net unrealized gains on available-for-sale securities (net of tax of $3 and $5, respectively)	18					18
Reclassification adjustment for gains included in net income (net of tax of $3 and $-, respectively)	0					0
Prior service cost and actuarial gains, net of amortization for defined benefit plans (net of tax of $3 and $-, respectively)	(3)					(3)
Total comprehensive income	476
Balance at Mar. 31, 2020	13,945	$ 120,836	$ 19,130	$ 687	(119,454)	146	(7,400)
Balance (shares) at Mar. 31, 2020		1	556	1,158
Other comprehensive income (loss), net of tax effect:
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	0
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Plan Amendments, Tax Effect	1
Accumulated deficit	(108,110)
Balance (shares) at Dec. 31, 2020		1	556	1,158
Balance at Dec. 31, 2020	25,259	$ 120,836	$ 19,130	$ 687	(108,110)	116	(7,400)								$ (1,139)	$ (1,139)
Comprehensive income:
Net income	4,993				4,993
Other comprehensive income (loss), net of tax effect:
Changes in net unrealized gains on available-for-sale securities (net of tax of $3 and $5, respectively)	(12)					(12)
Reclassification adjustment for gains included in net income (net of tax of $3 and $-, respectively)	(11)					(11)
Prior service cost and actuarial gains, net of amortization for defined benefit plans (net of tax of $3 and $-, respectively)	(4)					(4)
Total comprehensive income	4,966
Balance at Mar. 31, 2021	30,225	$ 120,836	$ 19,130	$ 687	$ (103,117)	$ 89	$ (7,400)
Balance (shares) at Mar. 31, 2021		1	556	1,158
Other comprehensive income (loss), net of tax effect:
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	3
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(3)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Plan Amendments, Tax Effect	(1)
Accumulated deficit	$ (103,117)