Consolidated Statement of Shareholder's Interest - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Total shareholder’s interest	$ 14,026		$ 14,347		$ 14,026	$ 14,347
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 14,347		$ 14,975	14,347	14,975	$ 15,884
Net income	637	21	441	603	1,924	2,228	2,224
Other comprehensive income/(loss), net of tax					372	44	(410)
Balance at end of period	14,026		14,347		14,026	14,347	14,975
Shareholder's Interest [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		5,227		5,227	5,227	5,227	5,227
Net income					0	0	0
Other comprehensive income/(loss), net of tax					0	0	0
Distributions declared					0	0	0
Balance at end of period	5,227		5,227		5,227	5,227	5,227
Accumulated Other Comprehensive Income/(Loss) (Note11) [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		(785)		(829)	(785)	(829)	(419)
Net income					0	0	0
Other comprehensive income/(loss), net of tax					372	44	(410)
Distributions declared					0	0	0
Balance at end of period	(413)		(785)		(413)	(785)	(829)
Retained Earnings [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 9,905		$ 10,577	9,905	10,577	11,076
Net income					1,924	2,228	2,224
Other comprehensive income/(loss), net of tax					0	0	0
Distributions declared					(2,415)	(2,900)	(2,723)
Balance at end of period	9,212		$ 9,905		9,212	9,905	10,577
Parent [Member]
Shareholder's Interest [Roll Forward]
Distributions declared					(2,415)	$ (2,900)	$ (2,723)
Accounting Standards Update 2016-13 [Member]
Total shareholder’s interest	(202)				(202)
Accounting Standards Update 2016-13 [Member] | Shareholder's Interest [Member]
Total shareholder’s interest	0				0
Accounting Standards Update 2016-13 [Member] | Accumulated Other Comprehensive Income/(Loss) (Note11) [Member]
Total shareholder’s interest	0				0
Accounting Standards Update 2016-13 [Member] | Retained Earnings [Member]
Total shareholder’s interest	$ (202)				$ (202)