Retirement and Deferred Compensation Plan - 401(k) Retirement Plan (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution percentage	50.00%
Percentage of maximum contributions per employee	6.00%
Contribution vesting period	5 years
Employer contribution amount	$ 3.3	$ 3.0	$ 2.6
Total matching contributions forfeited by plan participants during period	$ 0.7	$ 0.6	$ 0.4
Minimum
Defined Contribution Plan Disclosure [Line Items]
Percentage of employee deferral or contribution	1.00%
Maximum
Defined Contribution Plan Disclosure [Line Items]
Percentage of employee deferral or contribution	80.00%

Defined Contribution Plan, Employee Deferral Rate

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- Definition

Defined Contribution Plan, Employee Forfeitures

+ References

No definition available.

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- Definition

Minimum period the individual is required to perform services to be fully vested under the deferred compensation arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 710
-SubTopic 10
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6409961&loc=d3e20487-108367

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

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- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

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