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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Sep. 30, 2017
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Unrealized Gross Losses $ (13,779) $ (18,442)
Less than 12 months, Fair Value 751,808 1,187,758
12 months or more, Unrealized Gross Losses (60,355) (3,851)
12 months or more, Fair Value 1,640,016 394,922
Total Unrealized Gross Losses (74,134) (22,293)
Total, Fair Value 2,391,824 1,582,680
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Unrealized Gross Losses 0 0
Less than 12 months, Fair Value 0 0
12 months or more, Unrealized Gross Losses (1,146) (577)
12 months or more, Fair Value 118,818 49,423
Total Unrealized Gross Losses (1,146) (577)
Total, Fair Value 118,818 49,423
Municipal bonds    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Unrealized Gross Losses (10)  
Less than 12 months, Fair Value 1,380  
12 months or more, Unrealized Gross Losses 0  
12 months or more, Fair Value 0  
Total Unrealized Gross Losses (10)  
Total, Fair Value 1,380  
U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Unrealized Gross Losses (64) (759)
Less than 12 months, Fair Value 43,310 24,400
12 months or more, Unrealized Gross Losses (1,963) (697)
12 months or more, Fair Value 120,006 96,195
Total Unrealized Gross Losses (2,027) (1,456)
Total, Fair Value 163,316 120,595
Equity securities    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Unrealized Gross Losses (9)  
Less than 12 months, Fair Value 491  
12 months or more, Unrealized Gross Losses 0  
12 months or more, Fair Value 0  
Total Unrealized Gross Losses (9)  
Total, Fair Value 491  
Agency pass-through certificates    
Debt Securities, Available-for-sale [Line Items]    
Less than 12 months, Unrealized Gross Losses (13,696) (17,683)
Less than 12 months, Fair Value 706,627 1,163,358
12 months or more, Unrealized Gross Losses (57,246) (2,577)
12 months or more, Fair Value 1,401,192 249,304
Total Unrealized Gross Losses (70,942) (20,260)
Total, Fair Value $ 2,107,819 $ 1,412,662