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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses $ (8,687) $ (1,289)
Less than 12 months, fair value 808,820 304,197
12 months or more, unrealized gross losses (33,494) (10,637)
12 months or more, fair value 1,299,904 553,487
Total unrealized gross losses (42,181) (11,926)
Total, fair value 2,108,724 857,684
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses (1,135) 0
Less than 12 months, fair value 68,822 0
12 months or more, unrealized gross losses (1,873) (2,582)
12 months or more, fair value 48,128 100,467
Total unrealized gross losses (3,008) (2,582)
Total, fair value 116,950 100,467
U.S. government and agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses (244) (11)
Less than 12 months, fair value 24,962 3,167
12 months or more, unrealized gross losses (2,224) (5,638)
12 months or more, fair value 187,760 220,613
Total unrealized gross losses (2,468) (5,649)
Total, fair value 212,722 223,780
Agency pass-through certificates    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 months, unrealized gross losses (7,308) (1,278)
Less than 12 months, fair value 715,036 301,030
12 months or more, unrealized gross losses (29,397) (2,417)
12 months or more, fair value 1,064,016 232,407
Total unrealized gross losses (36,705) (3,695)
Total, fair value $ 1,779,052 $ 533,437