Fair Value Measurements - Schedule of Fair Values for Balance Sheet Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Liabilities
Unamortized discount on acquired loans	$ 13,500	$ 13,000
Level 1 | Carrying Amount
Assets
Cash and cash equivalents	282,638	298,696
Level 1 | Fair Value
Assets
Cash and cash equivalents	282,638	298,696
Level 3 | Carrying Amount
Assets
Loans, net, excluding fair valued loans, loans held for sale and impaired loans ¹	8,731,128	8,357,065
Level 3 | Fair Value
Assets
Loans, net, excluding fair valued loans, loans held for sale and impaired loans ¹	8,721,910	8,231,829
Level 2 | Carrying Amount
Liabilities
Time deposits	2,087,558	1,847,078
FHLB advances and other borrowings	275,000	275,000
Securities sold under repurchase agreements	62,537	90,907
Subordinated debentures and subordinated notes payable	108,528	108,468
Level 2 | Fair Value
Liabilities
Time deposits	2,086,538	1,848,550
FHLB advances and other borrowings	280,374	275,797
Securities sold under repurchase agreements	62,537	90,907
Subordinated debentures and subordinated notes payable	$ 102,285	$ 107,841