Income Tax Provision - Provision for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Current
Federal									$ 136	$ 73	$ (25)
State									2	(33)	(15)
Foreign									7	9	9
Total									145	49	(31)
Deferred
Federal									(9)	420	644
State									5	111	117
Foreign									1	0	(1)
Total									(3)	531	760
Provision for income taxes	$ (70)	$ (29)	$ 95	$ 146	$ 139	$ 210	$ 161	$ 70	$ 142	$ 580	$ 729