Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Numerator
Net income	$ 27	$ 65	$ 63	$ 53	$ 45	$ 64	$ 67	$ (10)	$ 208	$ 166	$ 304
Denominator
Weighted-average shares common shares outstanding (in shares)									189	197	204
Effect of potentially dilutive securities (in shares)									0	0	0
Weighted-average diluted shares outstanding (in shares)									189	197	204
Earnings per share
Earnings per share, basic (usd per share)	$ 0.14	$ 0.34	$ 0.33	$ 0.28	$ 0.23	$ 0.33	$ 0.33	$ (0.05)	$ 1.10	$ 0.84	$ 1.49
Earnings per share, diluted (usd per share)	$ 0.14	$ 0.34	$ 0.33	$ 0.28	$ 0.23	$ 0.33	$ 0.33	$ (0.05)	$ 1.10	$ 0.84	$ 1.49