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Stock-Based Compensation (Summary of Activity Relating to Restricted Units) (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Document Period End Date	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	13,116		19,144
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	2 years 8 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 17.63	$ 17.16
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 20
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	13,116	20,504
Restricted Units, Outstanding [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term	2 years 8 months 12 days	2 years 6 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	$ 20	$ 100
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 17.63		$ 17.31
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	1,500
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 12.79
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period	4,528
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price	$ 17.89
Closing market value of common stock	15.97
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price for restricted units, vested	$ 0.001
Number of Shares Underlying Restricted Units — Contingent Awards
Restricted Units, Outstanding [Roll Forward]
Outstanding at September 30, 2018	3,039,568
Granted	1,342,836
Earned/released	(1,386,735)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Number of Shares, Period Increase (Decrease)	296,759
Forfeited	(688,835)
Outstanding at June 30, 2019	2,010,075
Weighted average remaining recognition period of outstanding restricted units	1 year 7 months 6 days
Unrecognized stock-based compensation expense of outstanding restricted units	$ 27,000
Aggregate intrinsic value of outstanding restricted units (b)	$ 32,100
Number of Shares Underlying Restricted Units — Time-Based Awards
Restricted Units, Outstanding [Roll Forward]
Outstanding at September 30, 2018	6,872,087
Granted	6,214,882
Earned/released	(5,220,312)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Number of Shares, Period Increase (Decrease)	296,759
Forfeited	(1,159,444)
Outstanding at June 30, 2019	7,003,972
Weighted average remaining recognition period of outstanding restricted units	1 year 7 months 6 days
Unrecognized stock-based compensation expense of outstanding restricted units	$ 62,200
Aggregate intrinsic value of outstanding restricted units (b)	$ 111,900