Stock-Based Compensation (Summary of Activity Relating to Restricted Units) (Detail) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Document Period End Date	Mar. 31, 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 11 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 17.63			$ 16.41
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 0.1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	13,116		21,939
Restricted Units, Outstanding [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term	2 years 11 months	2 years 7 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	$ 0.1			$ 0.1
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	16.93
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	1,332,963
Earned/released	(1,366,735)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Number of Shares, Period Increase (Decrease)	296,759
Forfeited	(613,805)
Outstanding March 31, 2019	2,095,232
Weighted average remaining recognition period of outstanding restricted units	1 year 10 months 20 days
Unrecognized stock-based compensation expense of outstanding restricted units	$ 30.4
Aggregate intrinsic value of outstanding restricted units (b)	$ 40.2
Number of Shares Underlying Restricted Units — Time-Based Awards
Restricted Units, Outstanding [Roll Forward]
Outstanding at September 30, 2018	6,872,087
Granted	5,913,256
Earned/released	(4,594,896)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Number of Shares, Period Increase (Decrease)	296,759
Forfeited	(937,614)
Outstanding March 31, 2019	7,549,592
Weighted average remaining recognition period of outstanding restricted units	1 year 8 months 20 days
Unrecognized stock-based compensation expense of outstanding restricted units	$ 71.2
Aggregate intrinsic value of outstanding restricted units (b)	$ 123.3