Deposits - Composition of Deposits (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Deposits [Abstract]
Noninterest-bearing demand	$ 2,586,743	$ 1,956,025
Interest-bearing demand	7,139,058	6,248,638
Time deposits, greater than $250,000	352,913	493,530
Time deposits, less than or equal to $250,000	930,065	1,602,146
Total deposits	11,008,779	10,300,339
Brokered deposits	$ 329,000	$ 706,500