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Shareholders' Equity (Narrative) (Details)
 9 Months Ended 12 Months Ended
Sep. 14, 2016
USD ($)
$ / shares
Sep. 30, 2016
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
Sep. 24, 2013
shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]            
number of years     2      
Dividends declared to shareholders | $     $ 18,000,000 $ 14,600,000 $ 12,900,000  
Stockholders' Equity Note, Stock Split Ratio, Numerator       1    
Stockholders' Equity Note, Stock Split Ratio, Denominator       2    
Share Price | $ / shares $ 31.75          
Stockholders' Equity Note, Stock Split, Reduction of Capital | $ $ 5,000       (5,000)  
Stock repurchased during period, shares     750      
Stockholders' Equity Note, Stock Split, Conversion Ratio   1.5 1.5      
Subsidiaries [Member]            
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]            
Dividends declared to shareholders | $     $ 28,100,000 $ 16,800,000 16,000,000  
Acadia Trust [Member]            
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]            
Dividends declared to shareholders | $         $ 0  
2012 Common Stock Repurchase Program [Member]            
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]            
Stock repurchase program, shares of common stock approved for repurchase           375,000
Stock repurchased during period, shares     750      
Shares repurchased, average cost per share | $ / shares     $ 26.57      
Share Repurchase Program Three [Member] [Domain]            
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]            
Stock repurchase program, shares of common stock approved for repurchase           775,000