Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands	Jun. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	51	41
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (3,045)	$ (804)
12 months or more, unrealized gross losses	(5,122)	(1,986)
Total unrealized gross losses	(8,167)	(2,790)
Less than 12 months, fair value	407,365	240,610
12 months or more, fair value	313,115	257,961
Total, fair value	720,480	498,571
Held-to-maturity securities
Less than 12 months, unrealized gross losses	(105)	0
12 months or more, unrealized gross losses	0	(337)
Total unrealized gross losses	(105)	(337)
Less than 12 months, fair value	6,907	0
12 months or more, fair value	0	115,182
Total, fair value	6,907	115,182
Less than 12 months, unrealized gross losses	(3,150)	(804)
12 months or more, unrealized gross losses	(5,122)	(2,323)
Total unrealized gross losses	(8,272)	(3,127)
Less than 12 months, fair value	414,272	240,610
12 months or more, fair value	313,115	373,143
Total fair value	727,387	613,753
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(200)	0
12 months or more, unrealized gross losses	(712)	(50)
Total unrealized gross losses	(912)	(50)
Less than 12 months, fair value	71,304	0
12 months or more, fair value	49,288	24,950
Total, fair value	120,592	24,950
Municipal bonds
Available-for-sale securities
Less than 12 months, unrealized gross losses	(53)
12 months or more, unrealized gross losses	0
Total unrealized gross losses	(53)
Less than 12 months, fair value	9,905
12 months or more, fair value	0
Total, fair value	9,905
U.S. government and agency securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(2,647)	(656)
12 months or more, unrealized gross losses	(3,832)	(960)
Total unrealized gross losses	(6,479)	(1,616)
Less than 12 months, fair value	276,942	152,715
12 months or more, fair value	160,648	77,391
Total, fair value	437,590	230,106
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(145)	(148)
12 months or more, unrealized gross losses	(578)	(976)
Total unrealized gross losses	(723)	(1,124)
Less than 12 months, fair value	49,214	87,895
12 months or more, fair value	103,179	155,620
Total, fair value	$ 152,393	$ 243,515