Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2023	Sep. 30, 2022
Statement of Financial Position [Abstract]
Amortized Cost	$ 5,618,695	$ 5,569,604
Foreign currencies, cost	$ 10,547	$ 7,021
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)	350,000,000	350,000,000
Common stock, shares issued (in shares)	170,144,126	170,895,670
Common stock, shares outstanding (in shares)	170,144,126	170,895,670