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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses $ (131) $ (18,442)
Less than 12 months, fair value 71,537 1,187,758
12 months or more, unrealized gross losses (23,764) (3,851)
12 months or more, fair value 1,423,603 394,922
Total unrealized gross losses (23,895) (22,293)
Total, fair value 1,495,140 1,582,680
Corporate debt securities    
Schedule of Available-for-sale Securities [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 (1,352) (577)
12 months or more, fair value 98,648 49,423
Total unrealized gross losses (1,352) (577)
Total, fair value 98,648 49,423
Municipal bonds due    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses 0  
Less than 12 months, fair value 0  
12 months or more, unrealized gross losses (4)  
12 months or more, fair value 2,347  
Total unrealized gross losses (4)  
Total, fair value 2,347  
U.S. government and agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses 0 (759)
Less than 12 months, fair value 0 24,400
12 months or more, unrealized gross losses (1,336) (697)
12 months or more, fair value 104,572 96,195
Total unrealized gross losses (1,336) (1,456)
Total, fair value 104,572 120,595
Agency pass-through certificates    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses (131) (17,683)
Less than 12 months, fair value 71,537 1,163,358
12 months or more, unrealized gross losses (21,072) (2,577)
12 months or more, fair value 1,218,036 249,304
Total unrealized gross losses (21,203) (20,260)
Total, fair value $ 1,289,573 $ 1,412,662