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, 2020	$ 14,503		$ 915	$ 2	$ 15	$ 13,571
Balance (ASU 2016-13) at Mar. 31, 2020		$ (218)					$ (218)
Net income	1,029		0	0	0	1,029
Other comprehensive income (loss), net of tax	11		0	0	11	0
Balance at Sep. 30, 2020	15,325		915	2	26	14,382
Balance at Jun. 30, 2020	14,671		915	2	27	13,727
Net income	655		0	0	0	655
Other comprehensive income (loss), net of tax	(1)		0	0	(1)	0
Balance at Sep. 30, 2020	15,325		915	2	26	14,382
Balance at Mar. 31, 2021	15,595		915	2	8	14,670
Net income	1,543		0	0	0	1,543
Other comprehensive income (loss), net of tax	7		0	0	7	0
Balance at Sep. 30, 2021	17,145		915	2	15	16,213
Balance at Jun. 30, 2021	16,525		915	2	17	15,591
Net income	622		0	0	0	622
Other comprehensive income (loss), net of tax	(2)		0	0	(2)	0
Balance at Sep. 30, 2021	$ 17,145		$ 915	$ 2	$ 15	$ 16,213