Share-Based Payments - Schedule of weighted average assumptions for valuation of stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Weighted average assumptions relating to the valuation of stock options
Proceeds from exercises of stock options	$ 2,792	$ 2,160	$ 1,350
Stock Option Awards
Weighted average assumptions relating to the valuation of stock options
Expected life (in years)	5 years	5 years	5 years
Expected volatility for the Company’s common stock	35.80%	30.50%	27.40%
Risk-free interest rate	1.70%	3.00%	2.10%
Dividend yield	0.00%	0.00%	0.00%
Weighted average fair value of the stock options issued (in dollars per share)	$ 5.66	$ 5.86	$ 4.84
Aggregate fair value of stock options	$ 2,700	$ 5,100	$ 7,700
Aggregate intrinsic value of options exercised	500	900	1,300
Proceeds from exercises of stock options	3,000	2,200	1,400
Tax benefit realized for the tax deductions of stock option exercises	100	$ 200	$ 300
Total unrecognized compensation expenses related to unvested awards	$ 4,900
Weighted average period for recognition of unvested awards	1 year 9 months 18 days

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of tax benefit from exercise of option under share-based payment arrangement.

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Amount of cash inflow from exercise of option under share-based payment arrangement.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Rate of weighted-average expected volatility for award under share-based payment arrangement.

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- Definition

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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- Definition

Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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