Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, unrealized gross losses	$ (7,401)	$ (804)
Less than 12 months, fair value	221,880	240,610
12 months or more, unrealized gross losses	(11,234)	(2,323)
12 months or more, fair value	425,486	373,143
Total unrealized gross losses	(18,635)	(3,127)
Total, fair value	647,366	613,753
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, unrealized gross losses	(1,620)	0
Less than 12 months, fair value	51,306	0
12 months or more, unrealized gross losses	(5,092)	(50)
12 months or more, fair value	75,094	24,950
Total unrealized gross losses	(6,712)	(50)
Total, fair value	126,400	24,950
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, unrealized gross losses	(5,699)	(656)
Less than 12 months, fair value	151,823	152,715
12 months or more, unrealized gross losses	(4,241)	(960)
12 months or more, fair value	165,987	77,391
Total unrealized gross losses	(9,940)	(1,616)
Total, fair value	317,810	230,106
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, unrealized gross losses	(82)	(148)
Less than 12 months, fair value	18,751	87,895
12 months or more, unrealized gross losses	(1,901)	(1,313)
12 months or more, fair value	184,405	270,802
Total unrealized gross losses	(1,983)	(1,461)
Total, fair value	$ 203,156	$ 358,697