Stockholders' Equity - Capital Stock, Cash Dividend and Share Repurchases (Details) $ / shares in Units, $ in Millions				1 Months Ended	3 Months Ended		6 Months Ended
	Nov. 14, 2018 $ / shares	Aug. 14, 2018 USD ($) $ / shares	Jun. 01, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2018 $ / shares	Sep. 30, 2018 USD ($) vote $ / shares shares	Sep. 30, 2017 USD ($)
Equity [Abstract]
Common stock, authorized (in shares) | shares					750,000,000		750,000,000
Common stock, par value (in dollars per share)					$ 0.01		$ 0.01
Preferred stock, authorized (in shares) | shares					1,000,000		1,000,000
Preferred stock, par value (in dollars per share)					$ 0.01		$ 0.01
Number of votes per common share | vote							1
Amount of cash dividend declared | $		$ 8		$ 8
Class of Stock [Line Items]
Cash dividend per common share (in dollars per share)		$ 0.05	$ 0.05		$ 0.05	$ 0.05
Payment for repurchase of common stock | $					$ 2		$ 21	$ 0
Number of shares authorized in share repurchase program | $			$ 400
Number of shares repurchased during period | shares							923,188
Value of stock repurchased during period | $					$ 23		$ 23
Stock repurchase average price (in dollars per share)					$ 24.38
Remaining shares authorized to be repurchased | shares					377,000,000		377,000,000
Subsequent event
Class of Stock [Line Items]
Cash dividend per common share (in dollars per share)	$ 0.05