Consolidated Statements of Shareholder's Equity - USD ($) $ in Millions	Total	Cumulative Effect Period of Adoption Adjustment	Capital stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Retained earnings	Retained earnings Cumulative Effect Period of Adoption Adjustment
Balance at Mar. 31, 2019	$ 13,578		$ 915	$ 2	$ 3	$ 12,658
Net income	999		0	0	0	999
Other comprehensive income (loss), net of tax	12		0	0	12	0
Balance at Dec. 31, 2019	14,589		915	2	15	13,657
Balance at Sep. 30, 2019	14,411		915	2	18	13,476
Net income	181		0	0	0	181
Other comprehensive income (loss), net of tax	(3)		0	0	(3)	0
Balance at Dec. 31, 2019	14,589		915	2	15	13,657
Balance at Mar. 31, 2020	14,503		915	2	15	13,571
Balance (ASU 2016-13) at Mar. 31, 2020		$ (218)					$ (218)
Net income	1,697		0	0	0	1,697
Other comprehensive income (loss), net of tax	11		0	0	11	0
Balance at Dec. 31, 2020	15,993		915	2	26	15,050
Balance at Sep. 30, 2020	15,325		915	2	26	14,382
Net income	668		0	0	0	668
Other comprehensive income (loss), net of tax	0		0	0	0	0
Balance at Dec. 31, 2020	$ 15,993		$ 915	$ 2	$ 26	$ 15,050