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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses $ (8,351) $ (6,229)
Less than 12 months, fair value 912,470 578,646
12 months or more, unrealized gross losses (39,605) (23,128)
12 months or more, fair value 1,422,702 1,297,408
Total unrealized gross losses (47,956) (29,357)
Total, fair value 2,335,172 1,876,054
Corporate bonds due    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses (108) (183)
Less than 12 months, fair value 32,986 72,862
12 months or more, unrealized gross losses (3,030) (3,399)
12 months or more, fair value 46,970 46,601
Total unrealized gross losses (3,138) (3,582)
Total, fair value 79,956 119,463
U.S. government and agency securities due    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses (5,062) (5,010)
Less than 12 months, fair value 243,329 336,243
12 months or more, unrealized gross losses (2,221) (1,452)
12 months or more, fair value 109,611 57,344
Total unrealized gross losses (7,283) (6,462)
Total, fair value 352,940 393,587
Agency pass-through certificates    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses (3,181) (1,036)
Less than 12 months, fair value 636,155 169,541
12 months or more, unrealized gross losses (34,354) (18,277)
12 months or more, fair value 1,266,121 1,193,463
Total unrealized gross losses (37,535) (19,313)
Total, fair value $ 1,902,276 $ 1,363,004