Income Taxes - Provision for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Current
Federal									$ 2,756	$ (19)	$ (55)
State									386	120	76
Foreign									11	8	4
Total									3,153	109	25
Deferred
Federal									(2,243)	145	207
State									(276)	(141)	(49)
Foreign									(2)	(2)	(1)
Total									(2,521)	2	157
Provision for income taxes	$ 86	$ 215	$ 218	$ 113	$ (185)	$ 34	$ 158	$ 104	$ 632	$ 111	$ 182