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Loans Receivable - Additional Information (Details) - USD ($)
 3 Months Ended 6 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Nonaccrual interest income recognized     $ 3,228,000    
Impaired, interest lost on nonaccrual loans     $ 1,073,000    
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     97.70%    
Troubled debt restructuring, amount $ 2,183,000 $ 1,712,000 $ 11,451,000 $ 4,278,000  
Basis point reduction, minimum     1.00%    
Basis point reduction, maximum     2.00%    
Single family residential loans as percentage of restructured loans 85.50%   85.50%    
Covered assets $ 54,626,000   $ 54,626,000   $ 67,914,000
Loans receivable, gross 12,540,254,000   12,540,254,000   12,201,387,000
FDIC indemnification asset 0   0   8,967,000
FDIC clawback liability 39,906,000   39,906,000   37,143,000
Covered loans          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Loans receivable, gross $ 50,777,000   $ 50,777,000   $ 61,810,000
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     $ 190,519,000