Debt - Open Market Purchases - Additional Information (Detail) - USD ($) $ in Millions		Sep. 30, 2017	Sep. 30, 2016	Mar. 11, 2016	Sep. 30, 2015
Debt Instrument [Line Items]
Outstanding senior notes	[1]	$ 2,811	$ 2,778
6.75% Senior Notes due 2022
Debt Instrument [Line Items]
Interest rate		6.75%	6.75%		6.75%
Acquisition Corp. | 6.75% Senior Notes due 2022
Debt Instrument [Line Items]
Open market purchases of senior debt				$ 25
Outstanding senior notes		$ 635		$ 660
Interest rate		6.75%		6.75%

[1] Principal amount of debt of $2.846 billion and $2.815 billion less unamortized discount of $6 million and $3 million and unamortized deferred financing costs of $29 million and $34 million at September 30, 2017 and September 30, 2016, respectively.