Stock-Based Compensation - Narrative (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stocks granted	951,266	1,208,954
Proceeds from employees to satisfy tax obligation	$ 3.7	$ 0.1
Shares purchased under plan (in shares)	0	0
Board of Director Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stocks granted	815	954
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	3 years
Restricted Stock, Performance Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stocks granted	588,338	783,750
Performance-based awards granted, percent	100.00%
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stocks granted	951,266
Awards granted (USD per share)	$ 13.93	$ 11.36
Fair value of restricted stock awards vested	$ 11.3	$ 13.4
Withholding taxes paid	3.8	$ 4.3
Unrecognized compensation cost	$ 25.2
Unrecognized compensation cost, estimated weighted average amortization period	2 years 1 month
Maximum \| Restricted Stock, Performance Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards granted, percent	200.00%
Minimum \| Restricted Stock, Performance Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based awards granted, percent		0.00%
Share-based Compensation Award, Tranche One \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	33.33%
Share-based Compensation Award, Tranche Two \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	33.33%
Share-based Compensation Award, Tranche Two \| Restricted Stock, Performance Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Measurement period	3 years
Share-based Compensation Award, Tranche Three \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage	33.33%
Restricted Stock Units (RSUs) \| Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Quarterly dividends (in shares)	815	954
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase price of common stock (percent)	85.00%
Company \| Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Withholding taxes paid	$ 0.1	$ 4.2

Proceeds Received from Employees Related to Tax Withholding for Share-based Compensation

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Measurement Period

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Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Performance-Based Units Awarded

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Unrecognized cost of unvested share-based compensation awards.

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-Publisher FASB
-Name Accounting Standards Codification
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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The number of shares purchased on the open market during the period for issuance to employees under the plan.

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Percentage of vesting of share-based compensation awards.

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Purchase price of common stock expressed as a percentage of its fair value.

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