Equity-Based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 $ / shares
Stock Options
Outstanding at beginning of period (shares) | shares	2,138,500
Granted (shares) | shares	2,297,916
Exercised (shares) | shares	(29,967)
Forfeited (shares) | shares	(81,916)
Outstanding at end of period (shares) | shares	4,324,533
Weighted Average Exercise Price
Outstanding at beginning of period, weighted average exercise price (in USD per share) | $ / shares	$ 18.04	$ 13.13
Granted, weighted average exercise price (in USD per share) | $ / shares	22.52
Exercised, weighted average exercise price (in USD per share) | $ / shares	13.00
Forfeited, weighted average exercise price (in USD per share) | $ / shares	17.62
Outstanding at end of period, weighted average exercise price (in USD per share) | $ / shares	18.04	$ 13.13
Granted, weighted average grant date fair value (in USD per share) | $ / shares	$ 7.15
Options exercisable at end of period (shares) | shares	638,761
Unrecognized compensation expense related to unvested options at end of period | $	$ 19,847
Period for recognition of unrecognized compensation cost	2 years 6 months