Stock-Based Compensation (Summary of Activity Relating to Restricted Units) (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Document Period End Date	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	14,616		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	$ 17.13			$ 16.41
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 0.0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	14,616		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 9 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	$ 0.0			$ 0.1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 17.13		$ 17.31
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	0
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 0.00
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	13.23
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	1,088,105
Earned/released	(1,104,380)
Forfeited	(516,911)
Outstanding at December 31, 2018	2,506,382
Weighted average remaining recognition period of outstanding restricted units	2 years
Unrecognized stock-based compensation expense of outstanding restricted units	$ 34.5
Aggregate intrinsic value of outstanding restricted units(a)	$ 33.2
Number of Shares Underlying Restricted Units — Time-Based Awards
Restricted Units, Outstanding [Roll Forward]
Outstanding at September 30, 2018	6,872,087
Granted	5,679,945
Earned/released	(4,048,965)
Forfeited	(475,729)
Outstanding at December 31, 2018	8,027,338
Weighted average remaining recognition period of outstanding restricted units	1 year 10 months 20 days
Unrecognized stock-based compensation expense of outstanding restricted units	$ 90.5
Aggregate intrinsic value of outstanding restricted units(a)	$ 106.3