Income Taxes - Components of Income Before Income Taxes and Reconciliation of Statutory Federal Income Tax (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income before income taxes
United States									$ 282	$ 433	$ 382
Non-U.S.									50	57	39
Income before income taxes	$ 57	$ 97	$ 94	$ 84	$ 177	$ 94	$ 109	$ 110	$ 332	$ 490	$ 421
U.S. statutory tax rate									24.50%	35.00%	35.00%
Income taxes computed at U.S. statutory tax rate									$ 81	$ 171	$ 147
Increase (decrease) in amount computed resulting from:
Unrecognized tax benefits									0	2	3
State taxes, net of federal benefit									14	17	16
International rate differential									0	(7)	(5)
Permanent items									(3)	(8)	(11)
Remeasurement of net deferred taxes									73	0	0
Deemed repatriation									4	0	0
Tax Matters Agreement activity									(2)	10	0
Other									(1)	1	(2)
Income tax expense	$ 2	$ 3	$ 2	$ 71					$ 166	$ 186	$ 148
Effective tax rate									50.00%	38.00%	35.20%