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, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Income before income taxes
United States									$ 212	$ 282	$ 433
Non-U.S.									53	50	57
Income before income taxes	$ 28	$ 85	$ 80	$ 72	$ 57	$ 97	$ 94	$ 84	$ 265	$ 332	$ 490
U.S. statutory tax rate									21.00%	24.50%	35.00%
Income taxes computed at U.S. statutory tax rate									$ 56	$ 81	$ 171
Increase (decrease) in amount computed resulting from:
Unrecognized tax benefits									5	0	2
State taxes, net of federal benefit									9	14	21
International rate differential									2	0	(7)
Permanent items									(3)	(3)	(8)
Remeasurement of net deferred taxes									(4)	73	0
Return-to-provision adjustments									(6)	0	1
Deemed repatriation									0	4	0
Change in valuation allowance									(4)	1	(4)
Tax Matters Agreement activity									1	(2)	10
Other									1	(2)	0
Income tax expense			$ (2)		$ 2	$ 3	$ 2	$ 71	$ 57	$ 166	$ 186
Effective tax rate									21.50%	50.00%	38.00%