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CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) (Parenthetical) - $ / shares
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Common stock, par value (in usd per share) $ 0.001   $ 0.001  
Treasury stock (in shares) 24,723,533   24,721,372  
Common stock        
Common stock, par value (in usd per share)     $ 0.001 $ 0.001
Options exercised (in shares) 131,324 156,211    
Conversion of common stock units (in shares) 30,368 15,981    
Conversion of restricted stock units (in shares) 170,406 293,002    
Retained earnings        
Cash dividends (in usd per share) $ 0.855 $ 0.825    
Treasury stock, at cost        
Treasury stock (in shares) 24,723,533 24,721,372 24,721,372 24,672,932
Treasury stock, acquisitions (in shares) 2,161 48,440