Income Taxes - Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Current
Federal									$ (55)	$ (45)	$ 136
State									76	(10)	2
Foreign									4	3	7
Total									25	(52)	145
Deferred
Federal									207	(2,625)	(9)
State									(49)	48	5
Foreign									(1)	0	1
Total									157	(2,577)	(3)
Provision (benefit) for income taxes	$ 4	$ 69	$ 87	$ 22	$ 26	$ (2,821)	$ 70	$ 96	$ 182	$ (2,629)	$ 142