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Share-Based Compensation Plans - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares authorized under employee stock purchase plan (in shares) 3,250,000      
Shares available for grant or issuance under share-based compensation plans (in shares) 1,461,316      
Compensation expense to be recognized $ 9,300,000      
Maximum vesting period 5 years      
Proceeds from exercise of stock options $ 0 $ 741,000 $ 3,836,000  
Tax benefit from exercise of stock options 3,900,000 6,200,000 1,600,000  
Excess tax benefit from share-based compensation 1,616,000 2,546,000 1,663,000  
Total intrinsic value of options exercised $ 2,300,000 $ 4,600,000 $ 4,600,000  
Options that were exercisable (in shares) 93,000 143,000 306,726  
Weighted average exercise price of exercisable options (in dollars per share) $ 24.48 $ 23.06 $ 23.42  
Total intrinsic value of options outstanding and exercisable $ 5,300,000 $ 6,800,000 $ 7,600,000  
Awards of Shares and Stock Units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense $ 9,900,000 $ 8,900,000 $ 8,800,000  
Stock Options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares granted (in shares) 0 0 0  
Compensation expense $ 0 $ 0 $ 0  
Options outstanding 93,000 143,000 306,726 566,774
Restricted Stock Units (RSUs)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares vested, but issuance deferred (in shares) 97,200      
Issuance deferral period 5 years      
Employee Stock Purchase Plan (in shares)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares authorized under employee stock purchase plan (in shares) 3,000,000