Stock-Based Compensation (Summary of Activity Relating to Restricted Units) (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Document Period End Date	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	9,978		11,302
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	2 years 1 month 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 17.18	$ 17.63
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	9,978	13,116
Restricted Units, Outstanding [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term	2 years 1 month 6 days	2 years 10 months 24 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	$ 0	$ 100
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 17.18		$ 20.04
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	3,207
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 17.18
Closing market value of common stock	$ 16.78
Number of Shares Underlying Restricted Units — Performance-Based Awards
Restricted Units, Outstanding [Roll Forward]
Outstanding at September 30, 2019	1,991,325
Granted	1,067,900
Earned/released	(303,198)
Forfeited	(418,834)
Outstanding at March 31, 2020	2,640,267
Weighted average remaining recognition period of outstanding Restricted Units	1 year 10 months 24 days
Unrecognized stock-based compensation expense of outstanding Restricted Units	$ 25,000
Aggregate intrinsic value of outstanding Restricted Units (a)	$ 44,300
Number of Shares Underlying Restricted Units — Time-Based Awards
Restricted Units, Outstanding [Roll Forward]
Outstanding at September 30, 2019	8,998,944
Granted	4,415,974
Earned/released	(5,821,353)
Forfeited	(1,324,501)
Outstanding at March 31, 2020	7,585,070
Weighted average remaining recognition period of outstanding Restricted Units	1 year 8 months 12 days
Unrecognized stock-based compensation expense of outstanding Restricted Units	$ 67,100
Aggregate intrinsic value of outstanding Restricted Units (a)	$ 127,300