Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	$ (5,612)	$ (6,229)
Less than 12 months, fair value	470,410	578,646
12 months or more, unrealized gross losses	(13,401)	(23,128)
12 months or more, fair value	1,366,075	1,297,408
Total unrealized gross losses	(19,013)	(29,357)
Total, fair value	1,836,485	1,876,054
Corporate bonds due
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(1,001)	(183)
Less than 12 months, fair value	24,000	72,862
12 months or more, unrealized gross losses	(1,899)	(3,399)
12 months or more, fair value	33,101	46,601
Total unrealized gross losses	(2,900)	(3,582)
Total, fair value	57,101	119,463
U.S. government and agency securities due
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(3,136)	(5,010)
Less than 12 months, fair value	138,259	336,243
12 months or more, unrealized gross losses	(2,208)	(1,452)
12 months or more, fair value	115,518	57,344
Total unrealized gross losses	(5,344)	(6,462)
Total, fair value	253,777	393,587
Agency pass-through certificates
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(1,475)	(1,036)
Less than 12 months, fair value	308,151	169,541
12 months or more, unrealized gross losses	(9,294)	(18,277)
12 months or more, fair value	1,217,456	1,193,463
Total unrealized gross losses	(10,769)	(19,313)
Total, fair value	$ 1,525,607	$ 1,363,004