Equity-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (shares)		4,240,695
Granted (shares)		1,094,000
Exercised (shares)		166,709
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period		(39,331)
Outstanding at end of period (shares)	5,128,655	5,128,655
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding at beginning of period, weighted average exercise price (in USD per share)	$ 21.08	$ 21.08	$ 18.33
Granted, weighted average exercise price (in USD per share)		30.99
Exercised, weighted average exercise price (in USD per share)		16.11
Forfeited, weighted average exercise price (in USD per share)		22.12
Outstanding at end of period, weighted average exercise price (in USD per share)	$ 21.08	21.08	$ 18.33
Granted, weighted average grant date fair value (in USD per share)		$ 8.58
Options exercisable at end of period (shares)	911,560	911,560
Unrecognized compensation expense related to unvested options at end of period	$ 21,484	$ 21,484
Period for recognition of unrecognized compensation cost		2 years 2 months 12 days
Fair value of stock options that vested	$ 1,477	$ 3,228

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 cost to be recognized for option under share-based payment arrangement.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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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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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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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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Number of share options (or share units) exercised during the current period.

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