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Investments, at Fair Value - Narrative (Details)
3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2025
USD ($)
underlying_investment
investment_fund
Sep. 30, 2024
USD ($)
Sep. 30, 2025
USD ($)
underlying_investment
investment_fund
Sep. 30, 2024
USD ($)
Mar. 31, 2025
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]          
Return of investments in alternative assets held by Customer ExAlt Trusts     $ 7,780,000 $ 12,547,000 $ 47,800,000
Income (loss) from equity method investments     $ 11,800,000    
Alternative investment funds | investment_fund 188   188    
Underlying investments | underlying_investment 522   522    
Percent of underlying investments in private companies 85.00%   85.00%    
Unrealized gain (loss) on investments in available-for-sale debt securities $ 92,000 $ 26,000 $ 92,000 5,000  
Debt securities, available-for-sale, loss 0 500,000 0 1,000,000.0  
Equity securities without readily determinable fair value 26,500,000   26,500,000   26,500,000
Equity impairment loss 0 $ 0 0 $ 0  
Other Public Companies | Class A common stock, par value $0.001 per share          
Fair Value Measurement Inputs and Valuation Techniques [Line Items]          
Fair value of common stock $ 4,400,000   $ 4,400,000   $ 4,100,000