Stock-based compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Shares
Number of options, Outstanding beginning balance (in shares)	2,119
Forfeited (in shares)	(116)
Exercised (in shares)	(384)
Number of options, Outstanding ending balance (in shares)	1,619	2,119
Number of options, Nonvested at September 30, 2022 (in shares)	3
Number of options, Exercisable (in shares)	1,616
Weighted average exercise price per share
Weighted average exercise price per share, Outstanding beginning balance (in dollars per share)	$ 24.18
Forfeited (in dollars per share)	38.41
Exercised (in dollars per share)	18.50
Weighted average exercise price per share, Outstanding ending balance (in dollars per share)	24.51	$ 24.18
Weighted average exercise price per share, Nonvested at September 30, 2022 (in dollars per share)	106.55
Weighted average exercise price per share, Exercisable (in dollars per share)	$ 24.35
Weighted average remaining contractual term (years)
Outstanding	4 years 3 months 7 days	5 years 3 months
Weighted average remaining contractual term (years), Nonvested at September 30, 2022	6 years 11 months 8 days
Weighted average remaining contractual term (years), Exercisable	4 years 3 months 7 days
Aggregate intrinsic value
Aggregate intrinsic value, outstanding beginning balance	$ 6,715
Exercised	9,202
Aggregate intrinsic value, Nonvested at September 30, 2022	0
Aggregate intrinsic value, exercisable	36,798
Aggregate intrinsic value, outstanding ending balance	$ 36,798	$ 6,715
Performance Stock Options
Shares
Number of options, Outstanding beginning balance (in shares)	289
Forfeited (in shares)	(17)
Number of options, Outstanding ending balance (in shares)	272	289
Number of options, Nonvested at September 30, 2022 (in shares)	30
Number of options, Exercisable (in shares)	242
Weighted average exercise price per share
Weighted average exercise price per share, Outstanding beginning balance (in dollars per share)	$ 60.82
Forfeited (in dollars per share)	77.10
Weighted average exercise price per share, Outstanding ending balance (in dollars per share)	59.83	$ 60.82
Weighted average exercise price per share, Nonvested at September 30, 2022 (in dollars per share)	31.29
Weighted average exercise price per share, Exercisable (in dollars per share)	$ 63.36
Weighted average remaining contractual term (years)
Outstanding	6 years 4 months 20 days	7 years 4 months 13 days
Weighted average remaining contractual term (years), Nonvested at September 30, 2022	7 years 6 months 25 days
Weighted average remaining contractual term (years), Exercisable	6 years 2 months 26 days
Aggregate intrinsic value
Aggregate intrinsic value, outstanding beginning balance	$ 0
Aggregate intrinsic value, Nonvested at September 30, 2022	417
Aggregate intrinsic value, exercisable	625
Aggregate intrinsic value, outstanding ending balance	$ 1,042	$ 0