Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	$ (23,074)	$ (1,289)
Less than 12 months, fair value	1,394,124	304,197
12 months or more, unrealized gross losses	(9,711)	(10,637)
12 months or more, fair value	553,205	553,487
Total unrealized gross losses	(32,785)	(11,926)
Total, fair value	1,947,329	857,684
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(257)	0
Less than 12 months, fair value	54,743	0
12 months or more, unrealized gross losses	(1,812)	(2,582)
12 months or more, fair value	68,147	100,467
Total unrealized gross losses	(2,069)	(2,582)
Total, fair value	122,890	100,467
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(452)	(11)
Less than 12 months, fair value	24,750	3,167
12 months or more, unrealized gross losses	(1,406)	(5,638)
12 months or more, fair value	147,589	220,613
Total unrealized gross losses	(1,858)	(5,649)
Total, fair value	172,339	223,780
Agency pass-through certificates
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(22,365)	(1,278)
Less than 12 months, fair value	1,314,631	301,030
12 months or more, unrealized gross losses	(6,493)	(2,417)
12 months or more, fair value	337,469	232,407
Total unrealized gross losses	(28,858)	(3,695)
Total, fair value	$ 1,652,100	$ 533,437