Financial Transactions with Off-Balance-Sheet Risk and Concentrations of Credit Risk - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Loss Contingencies [Line Items]
Derivative (liabilities), at fair value, net		$ (2,600)
Occupancy expense, net	$ 3,382	14,547	$ 13,315	$ 12,798
Unused lines of Credit
Loss Contingencies [Line Items]
Fair value disclosure, off-balance sheet risk, amount, liability	679,400	714,600
Mortgages
Loss Contingencies [Line Items]
Fair value disclosure, off-balance sheet risk, amount, liability		8,100
Letter of Credit
Loss Contingencies [Line Items]
Fair value disclosure, off-balance sheet risk, amount, liability	11,600	7,000
Bank Facilities
Loss Contingencies [Line Items]
Occupancy expense, net	$ 1,100	$ 4,600	$ 4,300	$ 4,000