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Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Mar. 09, 2017	Mar. 08, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of vesting in first anniversary	30.00%
Percentage of vesting in second anniversary	30.00%
Percentage of vesting in third anniversary	40.00%
Net stock-based compensation expense	$ 16	$ 10	$ 10
Recognized stock-based compensation expense in connection with the modification of outstanding equity awards			5
Closing common stock price	$ 62.72
(Provision) benefit for income taxes	$ 193	33	33
Stock Options, Weighted Average Grant Date Fair Value, Granted	$ 15.21
Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price equal to market price on the date of grant	100.00%
Vesting period (in years)	3 years
Expiration period, years	10 years
Unrecognized compensation cost	$ 2
Weighted-average period, years	1 year
Intrinsic value of options exercised	$ 11	16	8
Cash received from exercises	22	$ 21	$ 8
(Provision) benefit for income taxes	$ 1
Stock Options, Weighted Average Grant Date Fair Value, Granted	$ 15.21	$ 12.76	$ 11.12
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)	3 years
Unrecognized compensation cost	$ 25
Weighted-average period, years	1 year
Estimated weighted average grant date fair value	$ 62.18	$ 51.03	$ 40.51
Intrinsic value of restricted stock units vested	$ 12	$ 7	$ 15
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)	3 years
2009 Long-Term Incentive Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, authorized shares					8,854,000
2017 Long-Term Incentive Plan [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, authorized shares				5,375,000
Supplemental 401(k) Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Supplemental retirement savings plan, aggregate value of the accounts, shares	116,000	109,000
Supplemental retirement savings plan aggregate value of the accounts paid in cash	$ 1	$ 2