Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2025	Sep. 30, 2024
Statement of Financial Position [Abstract]
Amortized cost	$ 8,672,620	$ 8,302,913
Foreign currencies, cost	13,763	7,973
Restricted foreign currencies, cost	$ 0	$ 1,219
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	1,000,000	1,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	500,000,000	350,000,000
Common stock, shares issued (in shares)	268,831,114	264,277,128
Common stock, shares outstanding (in shares)	268,831,114	264,277,128