Income Taxes - Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Current
Federal									$ (19)	$ (55)	$ (45)
State									120	76	(10)
Foreign									8	4	3
Total									109	25	(52)
Deferred
Federal									145	207	(2,625)
State									(141)	(49)	48
Foreign									(2)	(1)	0
Total									2	157	(2,577)
Provision (benefit) for income taxes	$ (185)	$ 34	$ 158	$ 104	$ 4	$ 69	$ 87	$ 22	$ 111	$ 182	$ (2,629)