Stockholders' Equity - Capital Stock, Cash Dividend and Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		3 Months Ended			6 Months Ended
	Nov. 12, 2019	Oct. 31, 2019	Jun. 30, 2018	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Class of Stock [Line Items]
Dividends declared but not yet paid				$ 10		$ 8	$ 10	$ 8
Dividends declared			$ 8	$ 10	$ 10	$ 8	$ 20	$ 16
Cash dividend per common share (in dollars per share)			$ 0.05	$ 0.06	$ 0.06	$ 0.05	$ 0.12	$ 0.10
Number of shares repurchased during period (in shares)							1,357,859
Repurchases of common stock							$ 32	$ 21
Remaining shares authorized to be repurchased (in shares)				$ 308			308
Stock-based compensation				$ 10	$ 5	$ 1	$ 15	$ 3
RSUs
Class of Stock [Line Items]
Shares granted (in shares)							558,000
Weighted average grant date fair value of shares (in dollars per share)							$ 23.17
PSUs
Class of Stock [Line Items]
Shares granted (in shares)							923,000
Weighted average grant date fair value of shares (in dollars per share)							$ 23.01
Subsequent Event
Class of Stock [Line Items]
Cash dividend per common share (in dollars per share)	$ 0.06
Repurchases of common stock		$ 1