Stock-Based Compensation - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2020		Sep. 30, 2019	Sep. 30, 2018	Mar. 09, 2017
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	$ 9,000,000		$ 8,000,000	$ 16,000,000
(Provision) benefit for income taxes	$ 191,000,000		70,000,000	193,000,000
Stock Options, Weighted Average Grant Date Fair Value, Granted	$ 10.68
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	$ 2,000,000
Weighted-average period, years	1 year
Options outstanding, aggregate intrinsic value	$ 0
Options exercisable, aggregate intrinsic value	$ 0
Closing stock price	$ 36.03
Intrinsic value of options exercised			1,000,000	11,000,000
Cash received from exercises	$ 1,000,000		$ 2,000,000	22,000,000
(Provision) benefit for income taxes				$ 1,000,000
Stock Options, Weighted Average Grant Date Fair Value, Granted	$ 10.68		$ 10.85	$ 15.21
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)	3 years
Unrecognized compensation cost	$ 10,000,000
Weighted-average period, years	1 year
Estimated weighted average grant date fair value	$ 49.36	[1]	$ 49.44	$ 62.18
Intrinsic value of restricted stock units vested	$ 13,000,000		$ 18,000,000	$ 12,000,000
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period (in years)	3 years
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	76,000		93,000
Supplemental retirement savings plan aggregate value of the accounts paid in cash			$ 1,000,000

[1] The number granted represents the number of shares issuable upon vesting of time-based restricted stock units and performance-based restricted stock units, assuming the Company performs at the target performance level in each year of the three-year performance period.