Segment Information - Research and Development and Selling, General and Administrative Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Stock compensation	$ 54,848	$ 67,097	$ 78,130
Depreciation and amortization	22,200	16,900	10,700
Total research and development expense	607,159	505,870	353,188
Total general and administrative expense	123,943	98,761	92,549
Research and development
Segment Reporting Information [Line Items]
Stock compensation	27,320	29,527	34,332
General and administrative expense
Segment Reporting Information [Line Items]
Stock compensation	27,528	37,570	43,798
Operating Segment
Segment Reporting Information [Line Items]
Candidate costs	347,571	259,280	162,459
R&D discovery costs	66,788	74,150	55,586
Salaries	109,085	96,418	73,668
Facilities related	29,233	25,782	16,267
Total research and development expense, excluding non-cash expense	552,677	455,630	307,980
Salaries	31,916	27,589	22,999
Professional, outside services, and other	53,589	24,733	20,720
Facilities related	5,625	4,116	3,415
Total general and administrative expense, excluding non-cash expense	91,130	56,438	47,134
Total research and development expense	607,159	505,870	353,188
Total general and administrative expense	123,943	98,761	92,549
Operating Segment | Research and development
Segment Reporting Information [Line Items]
Stock compensation	32,582	33,586	34,332
Depreciation and amortization	21,900	16,654	10,876
Operating Segment | General and administrative expense
Segment Reporting Information [Line Items]
Stock compensation	30,785	40,382	43,798
Depreciation and amortization	$ 2,028	$ 1,941	$ 1,617