Stock-based compensation expense - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2023 USD ($) $ / shares shares	Sep. 30, 2022 USD ($) $ / shares shares
Shares
Beginning balance (in shares) | shares	2,453,000
Granted (in shares) | shares	0
Forfeited (in shares) | shares	(71,000)
Exercised (in shares) | shares	(58,000)
Ending balance (in shares) | shares	2,324,000	2,453,000
Number of options, Nonvested at December 31, 2022 (in shares) | shares	290,000
Number of options, Exercisable (in shares) | shares	2,034,000
Weighted average exercise price per share
Beginning balance (in usd per share) | $ / shares	$ 24.67
Granted (in usd per share) | $ / shares	0
Forfeited (in usd per share) | $ / shares	35.15
Exercised (in dollars per share) | $ / shares	14.86
Ending balance (in usd per share) | $ / shares	24.59	$ 24.67
Weighted average exercise price per share, Nonvested at December 31, 2022 (in dollars per share) | $ / shares	36.46
Vested and exercisable (in usd per share) | $ / shares	$ 22.90
Weighted average remaining contractual term (years)
Outstanding	5 years 9 months 10 days	6 years 3 months 29 days
Weighted average remaining contractual term (years), Nonvested at December 31, 2022	6 years 8 months 26 days
Weighted average remaining contractual term (years), Exercisable	5 years 7 months 24 days
Aggregate Intrinsic Value
Aggregate intrinsic value, outstanding beginning balance | $	$ 33,447
Exercised | $	639
Aggregate intrinsic value, outstanding ending balance | $	4,250	$ 33,447
Aggregate intrinsic value, nonvested | $	100
Aggregate intrinsic value, Vested and exercisable | $	$ 4,150
Performance Stock Options
Shares
Beginning balance (in shares) | shares	312,000
Vested (in shares) | shares	(237,000)
Ending balance (in shares) | shares	312,000	312,000
Number of options, Nonvested at December 31, 2022 (in shares) | shares	56,000	293,000
Number of options, Exercisable (in shares) | shares	256,000	19,000
Weighted average exercise price per share
Beginning balance (in usd per share) | $ / shares	$ 61.35
Vested (in usd per share) | $ / shares	70.84
Ending balance (in usd per share) | $ / shares	61.35	$ 61.35
Weighted average exercise price per share, Nonvested at December 31, 2022 (in dollars per share) | $ / shares	31.29	63.27
Vested and exercisable (in usd per share) | $ / shares	$ 67.95	$ 31.29
Weighted average remaining contractual term (years)
Outstanding	7 years 9 months 29 days	8 years 3 months 29 days
Weighted average remaining contractual term (years), Nonvested at December 31, 2022	9 years 25 days	8 years 3 months
Weighted average remaining contractual term (years), Exercisable	7 years 6 months 21 days	9 years 6 months 25 days
Aggregate Intrinsic Value
Aggregate intrinsic value, outstanding beginning balance | $	$ 296
Aggregate intrinsic value, outstanding ending balance | $		$ 296
Aggregate intrinsic value, nonvested | $		222
Aggregate intrinsic value, Vested and exercisable | $		$ 74