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Allowance for Losses on Loans - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2017
Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Charge-offs, net of (recoveries) $ (3,082) $ (5,296)    
Loans receivable, gross $ 12,456,216   $ 12,201,387  
Ratio of past due loans to total loans, net of charge-offs and LIPs 0.43%   0.40%  
Allowance for loan losses $ 127,155 $ 118,456 $ 123,073 $ 113,494
Allowance for credit losses and reserve for unfunded commitments $ 133,905      
Allowance for credit losses and reserve for unfunded commitments, percent of gross loans 1.08%      
Non-Accrual loans        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Nonaccrual loans $ 45,520   49,580  
Loans Collectively Evaluated for Impairment        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Loans receivable, gross 11,193,552   11,000,212  
Allowance for loan losses 126,520   122,947  
Loans Individually Evaluated for Impairment        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Loans receivable, gross 66,888   28,553  
Allowance for loan losses 635   126  
Unfunded Loan Commitment        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Allowance for loan losses 6,750   7,750  
Non-Performing Loans        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Loans receivable, gross $ 63,448   $ 70,238  
Ratio of non-performing assets to total assets 0.41%   0.46%  
Nonaccrual loans $ 45,520   $ 49,580