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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Sep. 30, 2019
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, unrealized gross losses $ (1,361) $ (804)
Less than 12 months, fair value 281,475 240,610
12 months or more, unrealized gross losses (2,886) (2,323)
12 months or more, fair value 331,510 373,143
Total unrealized gross losses (4,247) (3,127)
Total, fair value 612,985 613,753
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, unrealized gross losses   0
Less than 12 months, fair value   0
12 months or more, unrealized gross losses   (50)
12 months or more, fair value   24,950
Total unrealized gross losses   (50)
Total, fair value   24,950
U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, unrealized gross losses (701) (656)
Less than 12 months, fair value 156,068 152,715
12 months or more, unrealized gross losses (1,627) (960)
12 months or more, fair value 122,155 77,391
Total unrealized gross losses (2,328) (1,616)
Total, fair value 278,223 230,106
Mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, unrealized gross losses (660) (148)
Less than 12 months, fair value 125,407 87,895
12 months or more, unrealized gross losses (1,259) (1,313)
12 months or more, fair value 209,355 270,802
Total unrealized gross losses (1,919) (1,461)
Total, fair value $ 334,762 $ 358,697