Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Before tax
Unrealized translation gain	$ 6	$ 10	$ (3)	$ 5
Amortization of unrecognized prior service credits included in net income	(3)	0	(6)	0
Total other comprehensive income	3	10	(9)	5
Tax benefit (expense)
Unrealized translation gain	0	(1)	0	0
Amortization of unrecognized prior service credits included in net income	1	0	2	0
Total other comprehensive income	1	(1)	2	0
Net of tax
Unrealized translation gain	6	9	(3)	5
Amortization of unrecognized prior service credits included in net income	(2)	0	(4)	0
Other comprehensive income (loss)	$ 4	$ 9	$ (7)	$ 5