Stock-based compensation expense - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023
Shares
Beginning balance (in shares)	2,119,000
Forfeited (in shares)	(85,000)
Exercised (in shares)	(58,000)
Ending balance (in shares)	1,976,000	2,119,000
Number of options, Nonvested at December 31, 2022 (in shares)	19,000
Number of options, Exercisable (in shares)	1,958,000
Weighted average exercise price per share
Beginning balance (in usd per share)	$ 24.18
Forfeited (in usd per share)	34.45
Exercised (in dollars per share)	16.42
Ending balance (in usd per share)	23.97	$ 24.18
Weighted average exercise price per share, Nonvested at December 31, 2022 (in dollars per share)	77.89
Vested and exercisable (in usd per share)	$ 23.46
Weighted average remaining contractual term (years)
Outstanding	1 year 10 months 9 days	5 years 3 months
Weighted average remaining contractual term (years), Nonvested at December 31, 2022	5 years 5 months 23 days
Weighted average remaining contractual term (years), Exercisable	1 year 9 months 25 days
Aggregate Intrinsic Value
Aggregate intrinsic value, outstanding beginning balance	$ 6,715
Exercised	483
Aggregate intrinsic value, nonvested	5
Aggregate intrinsic value, Vested and exercisable	29,946
Aggregate intrinsic value, outstanding ending balance	$ 29,951	$ 6,715
Performance Stock Options
Shares
Beginning balance (in shares)	289,000
Forfeited (in shares)	(12,000)
Ending balance (in shares)	277,000	289,000
Number of options, Nonvested at December 31, 2022 (in shares)	30,000	30,000
Number of options, Exercisable (in shares)	247,000	259,000
Weighted average exercise price per share
Beginning balance (in usd per share)	$ 60.82
Forfeited (in usd per share)	67.85
Ending balance (in usd per share)	60.53	$ 60.82
Weighted average exercise price per share, Nonvested at December 31, 2022 (in dollars per share)	31.29	31.29
Vested and exercisable (in usd per share)	$ 64.07	$ 64.24
Weighted average remaining contractual term (years)
Outstanding	7 years 1 month 17 days	7 years 4 months 13 days
Weighted average remaining contractual term (years), Nonvested at December 31, 2022	8 years 3 months 25 days	8 years 6 months 25 days
Weighted average remaining contractual term (years), Exercisable	6 years 11 months 26 days	7 years 2 months 23 days
Aggregate Intrinsic Value
Aggregate intrinsic value, outstanding beginning balance	$ 0
Aggregate intrinsic value, nonvested		$ 0
Aggregate intrinsic value, Vested and exercisable	251	0
Aggregate intrinsic value, Nonvested at December 31, 2022	167
Aggregate intrinsic value, outstanding ending balance	$ 418	$ 0