FHLB Advances and Other Borrowings - Schedule of Advances, Related Party Notes (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Debt Instrument [Line Items]
Total	$ 721,000	$ 643,200
Fair value adjustment	9	14
Total FHLB advances and other borrowings	721,009	643,214
Notes payable to banks
Debt Instrument [Line Items]
Federal Home Loan Bank (FHLB) notes payable and fed funds advance	$ 56,000	56,000
Notes payable to banks | Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	1.05%
Notes payable to banks | Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	3.66%
Notes payable to banks
Debt Instrument [Line Items]
Federal Home Loan Bank (FHLB) notes payable and fed funds advance	$ 665,000	512,200
Notes payable to banks | Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	1.42%
Notes payable to banks | Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	1.54%
Federal Home Loan Bank fed funds advance
Debt Instrument [Line Items]
Interest rate (as a percent)	1.33%
Federal Home Loan Bank (FHLB) notes payable and fed funds advance	$ 0	$ 75,000