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Available-for-Sale Securities - Interest-bearing Investments in Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2023
Sep. 30, 2022
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 547,008 $ 533,762
Gross Unrealized Gains 0 0
Gross Unrealized Losses 16,203 22,797
Fair Value 530,805 510,965
Money Market Funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 308,354 266,362
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 308,354 266,362
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 160,370 183,266
Gross Unrealized Gains 0 0
Gross Unrealized Losses 10,060 14,958
Fair Value 150,310 168,308
U.S. Government Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 44,782 44,658
Gross Unrealized Gains 0 0
Gross Unrealized Losses 3,644 4,429
Fair Value 41,138 40,229
Municipal Bond [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 7,513 17,759
Gross Unrealized Gains 0 0
Gross Unrealized Losses 417 1,634
Fair Value 7,096 16,125
Supranational and Sovereign Debt [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 18,566 11,882
Gross Unrealized Gains 0 0
Gross Unrealized Losses 1,774 1,288
Fair Value 16,792 10,594
Asset Backed Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 7,423 9,835
Gross Unrealized Gains 0 0
Gross Unrealized Losses 308 488
Fair Value $ 7,115 $ 9,347