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Fair Value Measurements - Securities in Unrealized Loss Positions (Details)
$ in Thousands
Jun. 30, 2022
USD ($)
security
Sep. 30, 2021
USD ($)
security
Debt Securities, Available-for-sale [Line Items]    
Number of debt securities with unrealized loss | security 149 31
Available-for-sale securities    
Less than 12 months, unrealized gross losses $ (60,478) $ (1,584)
12 months or more, unrealized gross losses (8,053) (1,056)
Total unrealized gross losses (68,531) (2,640)
Less than 12 months, fair value 1,719,624 52,397
12 months or more, fair value 135,304 102,324
Total, fair value 1,854,928 154,721
Held-to-maturity securities    
Less than 12 months, unrealized gross losses (30,170) 0
Less than 12 months, fair value 445,576 0
12 months or more, unrealized gross losses 0 0
12 months or more, fair value 0 0
Total unrealized gross losses (30,170) 0
Total, fair value 445,576 0
Less than 12 months, unrealized gross losses (90,648) (1,584)
Less than 12 months, fair value 2,165,200 52,397
12 months or more, unrealized gross losses (8,053) (1,056)
12 months or more, fair value 135,304 102,324
Total unrealized gross losses (98,701) (2,640)
Total fair value 2,300,504 154,721
Corporate debt securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (19,238)  
12 months or more, unrealized gross losses 0  
Total unrealized gross losses (19,238)  
Less than 12 months, fair value 226,780  
12 months or more, fair value 0  
Total, fair value 226,780  
Municipal bonds due    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (679)  
12 months or more, unrealized gross losses 0  
Total unrealized gross losses (679)  
Less than 12 months, fair value 14,960  
12 months or more, fair value 0  
Total, fair value 14,960  
U.S. government and agency securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (1,122)  
12 months or more, unrealized gross losses 0  
Total unrealized gross losses (1,122)  
Less than 12 months, fair value 45,462  
12 months or more, fair value 0  
Total, fair value 45,462  
Asset-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (10,232) (15)
12 months or more, unrealized gross losses (1,935) (614)
Total unrealized gross losses (12,167) (629)
Less than 12 months, fair value 651,065 4,639
12 months or more, fair value 77,426 70,058
Total, fair value 728,491 74,697
Mortgage-backed securities    
Available-for-sale securities    
Less than 12 months, unrealized gross losses (29,207) (1,569)
12 months or more, unrealized gross losses (6,118) (442)
Total unrealized gross losses (35,325) (2,011)
Less than 12 months, fair value 781,357 47,758
12 months or more, fair value 57,878 32,266
Total, fair value $ 839,235 $ 80,024