Fair Value Measurements - Schedule of Fair Values for Balance Sheet Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Level 1 | Carrying Amount
Assets
Cash and cash equivalents	$ 270,168	$ 524,611
Level 1 | Fair Value
Assets
Cash and cash equivalents	270,168	524,611
Level 3 | Carrying Amount
Assets
Loans, net excluding fair valued loans and loans held for sale	7,624,789	7,473,973
Level 3 | Fair Value
Assets
Loans, net excluding fair valued loans and loans held for sale	7,567,288	7,433,851
Level 2 | Carrying Amount
Assets
Accrued interest receivable	49,357	49,531
Cash surrender value of life insurance policies	29,387	29,166
Federal Home Loan Bank stock	40,513	47,025
Liabilities
Deposits	8,706,247	8,604,790
FHLB advances and other borrowings	711,029	871,037
Securities sold under repurchase agreements	142,741	141,688
Accrued interest payable	4,592	4,074
Subordinated debentures and subordinated notes payable	108,178	111,873
Level 2 | Fair Value
Assets
Accrued interest receivable	49,357	49,531
Cash surrender value of life insurance policies	29,387	29,166
Federal Home Loan Bank stock	40,513	47,025
Liabilities
Deposits	8,705,302	8,603,708
FHLB advances and other borrowings	713,227	874,763
Securities sold under repurchase agreements	142,741	141,688
Accrued interest payable	4,592	4,074
Subordinated debentures and subordinated notes payable	$ 108,183	$ 112,826