Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	$ (36,439)	$ (1,289)
Less than 12 months, fair value	1,838,706	304,197
12 months or more, unrealized gross losses	(10,556)	(10,637)
12 months or more, fair value	720,796	553,487
Total unrealized gross losses	(46,995)	(11,926)
Total, fair value	2,559,502	857,684
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(1,134)	0
Less than 12 months, fair value	48,866	0
12 months or more, unrealized gross losses	(3,411)	(2,582)
12 months or more, fair value	76,546	100,467
Total unrealized gross losses	(4,545)	(2,582)
Total, fair value	125,412	100,467
Municipal bonds due
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(26)
Less than 12 months, fair value	3,639
12 months or more, unrealized gross losses	0
12 months or more, fair value	0
Total unrealized gross losses	(26)
Total, fair value	3,639
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(743)	(11)
Less than 12 months, fair value	24,470	3,167
12 months or more, unrealized gross losses	(3,634)	(5,638)
12 months or more, fair value	212,717	220,613
Total unrealized gross losses	(4,377)	(5,649)
Total, fair value	237,187	223,780
Agency pass-through certificates
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, unrealized gross losses	(34,536)	(1,278)
Less than 12 months, fair value	1,761,731	301,030
12 months or more, unrealized gross losses	(3,511)	(2,417)
12 months or more, fair value	431,533	232,407
Total unrealized gross losses	(38,047)	(3,695)
Total, fair value	$ 2,193,264	$ 533,437