Stock-based Compensation - Narrative (Details)		1 Months Ended	3 Months Ended	12 Months Ended
	Sep. 26, 2018 shares	Feb. 28, 2025 shares	Sep. 30, 2025 USD ($) shares	Sep. 30, 2025 USD ($) shares	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($)	Aug. 22, 2023 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized (in shares) | shares				249,000
Aggregate number of common stock shares reserved for issuance (in shares) | shares			716,000	716,000	611,000
Capitalized computer software, net			$ 3,200,000	$ 3,200,000	$ 5,200,000
Share-based liabilities settled						$ 5,900,000
Total stock-based compensation expense				$ 64,459,000	50,925,000	30,278,000
Employer matching percentage				50.00%
Percentage of employee's gross pay matched				6.00%
Employer matching contributions				$ 3,000,000.0	2,600,000
Share-based Payment Arrangement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capitalized computer software, net			1,300,000	$ 1,300,000	1,300,000
Restricted Stock Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Conversion ratio				1
Unrecognized compensation cost			64,800,000	$ 64,800,000
Recognize cost weighted average period				2 years 8 months 12 days
Grant date fair value				$ 57,900,000	42,700,000	32,200,000
Vested in period, fair value				$ 37,000,000	31,000,000	33,700,000
Restricted Stock Unit | Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				1 year
Performance Stock Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost			$ 37,800,000	$ 37,800,000
Recognize cost weighted average period				1 year 7 months 6 days
Grant date fair value				$ 66,900,000	11,500,000	21,200,000
Vested in period, fair value				$ 14,000,000	6,900,000	1,800,000
Performance Stock Unit | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of PSUs that may vest based on performance				0.00%
Performance Stock Unit | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of PSUs that may vest based on performance				140.00%
Performance Stock Unit | Employee | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			10 months
Performance Stock Unit | Employee | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				34 months
Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Unrecognized compensation cost			$ 0	$ 0
Recognize cost weighted average period				2 months 12 days
Award term				10 years
Expected dividend yield				0.00%
Exercises in period, intrinsic value					9,200,000	1,100,000
2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized (in shares) | shares		3,700,000
Maximum aggregate number of shares (in shares) | shares	10,068,911
Aggregate number of common stock shares reserved for issuance (in shares) | shares			5,260,083	5,260,083
Inducement Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized (in shares) | shares		700,000
Maximum aggregate number of shares (in shares) | shares							1,400,000
2018 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of common stock shares reserved for issuance (in shares) | shares	275,225
Minimum annual increase in share reserved for issuance | shares	249,470
Annual automatic Increase in share reserved for issuance	1.00%
Percentage of payroll deduction to purchase common stock	15.00%
ESPP eligible employee common stock purchase price ratio	85.00%
2018 ESPP | Shares subject to options to purchase common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense				$ 2,400,000	$ 1,600,000	$ 0