FHLB Advances and Other Borrowings - Summary of Advances and Other Borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Total	$ 275,000	$ 643,200
Fair value adjustment	0	14
Total FHLB advances and other borrowings	275,000	643,214
Notes payable to banks
Debt Instrument [Line Items]
Federal Home Loan Bank (FHLB) notes payable and fed funds advance	$ 175,000	$ 56,000
Notes payable to banks | Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	1.05%	0.53%
Notes payable to banks | Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	3.66%	3.66%
Notes payable to banks
Debt Instrument [Line Items]
Federal Home Loan Bank (FHLB) notes payable and fed funds advance	$ 0	$ 512,200
Notes payable to banks | Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	1.25%
Notes payable to banks | Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	1.27%
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	$ 100,000	$ 75,000