Loans Receivable - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual interest income recognized				$ 3,926
Impaired, interest lost on nonaccrual loans				$ 1,741
Ratio of past due loans to total loans, net of charge-offs and LIPs		0.40%		0.40%		0.40%
Percent of TDRs classified as performing				96.70%
Troubled debt restructuring, amount		$ 789	$ 1,836	$ 12,240	$ 6,114
Basis point reduction, minimum				1.00%
Basis point reduction, maximum				2.00%
Single family residential loans as percentage of restructured loans		89.40%		89.40%
Payments to settle FDIC clawback liability	$ 39,900
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term for payment and rate reduction				6 months
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term for payment and rate reduction				24 months
Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring, amount				$ 171,603