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Short-Term Borrowings and Long-Term Debt Subordinated Debt (Details)
1 Months Ended 3 Months Ended 12 Months Ended
Jun. 30, 2015
Sep. 30, 2016
USD ($)
Dec. 31, 2017
USD ($)
subsidiarytrusts
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Sep. 30, 2017
Mar. 31, 2017
USD ($)
Nov. 30, 2014
USD ($)
May 31, 2007
USD ($)
Subordinated Debt [Abstract]                  
Amortization of financing costs     $ 845,000 $ 617,000 $ 582,000        
Net Income     171,753,000 161,625,000 149,502,000        
Income taxes     $ 62,701,000 $ 46,624,000 $ 49,921,000        
Subsidiary trusts owned by parent | subsidiarytrusts     3            
Junior subordinated deferrable interest debentures                  
Subordinated Debt [Abstract]                  
Effective interest rate           6.52%      
Extinguishment of debt, amount   $ 150,000,000              
Amortization of financing costs   1,800,000              
Non-Interest Income (Expense) | Junior subordinated deferrable interest debentures                  
Subordinated Debt [Abstract]                  
Loss on debt extinguishment   (5,600,000)              
Unrealized Effective Portions of Losses on Forward-Starting Interest Rate Swaps | Reclassification out of Accumulated Other Comprehensive Income | Junior subordinated deferrable interest debentures                  
Subordinated Debt [Abstract]                  
Net Income   2,500,000              
Income taxes   1,300,000              
Senior Notes                  
Subordinated Debt [Abstract]                  
Debt instrument, face amount             $ 125,000,000    
Effective interest rate             3.95%    
Stated interest rate             3.60%    
Junior subordinated deferrable interest debentures                  
Subordinated Debt [Abstract]                  
Subordinated Debt     $ 16,496,000            
Junior subordinated deferrable interest debentures | Columbia Bancorp Statutory Trust                  
Subordinated Debt [Abstract]                  
Interest Rate     3.99%            
Subordinated Debt     $ 6,186,000            
Maturity     Jun. 30, 2034            
Callable     Mar. 31, 2018            
Call Price     1.000            
Junior subordinated deferrable interest debentures | Columbia Bancorp Statutory Trust II                  
Subordinated Debt [Abstract]                  
Interest Rate     3.48%            
Subordinated Debt     $ 4,124,000            
Maturity     Mar. 15, 2035            
Callable     Mar. 31, 2018            
Call Price     1.000            
Junior subordinated deferrable interest debentures | Columbia Bancorp Statutory Trust III                  
Subordinated Debt [Abstract]                  
Interest Rate     3.36%            
Subordinated Debt     $ 6,186,000            
Maturity     Jun. 15, 2035            
Callable     Mar. 31, 2018            
Call Price     1.000            
Subordinated debt                  
Subordinated Debt [Abstract]                  
Debt instrument, face amount   $ 150,000,000              
Debt instrument, term 10 years                
Interest Rate 4.50%                
Effective interest rate   4.69%              
Subordinated debt | May 2017 Subordinated Debt                  
Subordinated Debt [Abstract]                  
Effective interest rate                 5.96%
Subordinated Debt                 $ 100,000,000
Subordinated debt | November 2024 Subordinated Debt                  
Subordinated Debt [Abstract]                  
Effective interest rate               4.87%  
Subordinated Debt               $ 100,000,000  
Stated interest rate               4.50%