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Fair Value Measurements - Securities in Unrealized Loss Positions (Details)
$ in Thousands
Mar. 31, 2022
USD ($)
security
Sep. 30, 2021
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 131 31
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (7,667) $ (1,584)
12 months or more, unrealized gross losses (15,704) (1,056)
Total unrealized gross losses (23,371) (2,640)
Less than 12 months, fair value 551,598 52,397
12 months or more, fair value 236,765 102,324
Total, fair value 788,363 154,721
Held-to-maturity securities    
Less than 12 months, unrealized gross losses (7,037) 0
Less than 12 months, fair value 291,911 0
12 months or more, unrealized gross losses 0 0
12 months or more, fair value 0 0
Total unrealized gross losses (7,037) 0
Total, fair value 291,911 0
Less than 12 months, unrealized gross losses (14,704) (1,584)
Less than 12 months, fair value 843,509 52,397
12 months or more, unrealized gross losses (15,704) (1,056)
12 months or more, fair value 236,765 102,324
Total unrealized gross losses (30,408) (2,640)
Total fair value 1,080,274 154,721
Corporate debt securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses 0  
12 months or more, unrealized gross losses (9,482)  
Total unrealized gross losses (9,482)  
Less than 12 months, fair value 0  
12 months or more, fair value 105,528  
Total, fair value 105,528  
Municipal bonds due    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (156)  
12 months or more, unrealized gross losses 0  
Total unrealized gross losses (156)  
Less than 12 months, fair value 5,610  
12 months or more, fair value 0  
Total, fair value 5,610  
U.S. government and agency securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (659)  
12 months or more, unrealized gross losses 0  
Total unrealized gross losses (659)  
Less than 12 months, fair value 34,697  
12 months or more, fair value 0  
Total, fair value 34,697  
Asset-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (1,269) (15)
12 months or more, unrealized gross losses (972) (614)
Total unrealized gross losses (2,241) (629)
Less than 12 months, fair value 277,125 4,639
12 months or more, fair value 67,792 70,058
Total, fair value 344,917 74,697
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (5,583) (1,569)
12 months or more, unrealized gross losses (5,250) (442)
Total unrealized gross losses (10,833) (2,011)
Less than 12 months, fair value 234,166 47,758
12 months or more, fair value 63,445 32,266
Total, fair value $ 297,611 $ 80,024