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Share-Based Compensation (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Grant Date Fair Value Assumptions [Abstract]      
Share-based compensation $ 12,482,000 $ 9,902,000 $ 9,543,000
Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 5 years    
Stock option activity [Roll Forward]      
Beginning balance (in shares) 2,550,883    
Granted (in shares) 108,275    
Exercised (in shares) 364,984 343,237 543,028
Forfeited (in shares) 6,775    
Ending balance (in shares) 2,287,399 2,550,883  
Exercisable (In Shares) 1,650,386    
Stock option weighted average exercise price [Abstract]      
Beginning balance (in dollars per share) $ 34.70    
Granted (in dollars per share) 116.99    
Exercised (in dollars per share) 26.88    
Forfeited (in dollars per share) 94.20    
Ending balance (in dollars per share) 39.67 $ 34.70  
Exercisable at end of period (in dollars per share) $ 26.33    
Weighted Average Remaining Contractual Term [Abstract]      
Weighted average remaining contractual term of shares outstanding (in years) 3 years 10 months 6 days    
Weighted average remaining contractual term of shares exercisable at end of period (in years) 2 years 6 months 19 days    
Aggregate Intrinsic Value [Abstract]      
Ending balance $ 205,820,051    
Exercisable at end of period $ 170,526,117    
Cash Proceeds and Tax Benefits [Abstract]      
Exercised (in shares) 364,984 343,237 543,028
Cash proceeds $ 9,809,000 $ 10,340,000 $ 17,137,000
Intrinsic value of options exercised 33,302,000 21,094,000 22,676,000
Tax benefits realized $ 12,809,000 $ 7,891,000 $ 8,326,000
Grant Date Fair Value Assumptions [Abstract]      
Expected volatility (in hundredths) 26.60% 29.70% 33.80%
Expected term (in years) 7 years 3 months 4 days 7 years 1 month 6 days 7 years 2 months 12 days
Risk-free interest rate (in hundredths) 2.44% 1.75% 1.95%
Expected dividend yield (in hundredths) 1.50% 1.50% 1.50%
Grant date fair value (in dollars per share) $ 32.00 $ 22.86 $ 22.57
Share-based compensation $ 3,553,000 $ 3,735,000 $ 3,688,000
Recognized tax benefits 888,000 1,409,000 1,331,000
Unamortized compensation expense $ 3,472,000    
Expense recognition over weighted average period (in years) 1 year 8 months 10 days    
Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 5 years    
Award performance period 3 years    
Shares available for grant 1,186,521    
Grant Date Fair Value Assumptions [Abstract]      
Share-based compensation $ 8,547,000 $ 5,993,000 $ 5,513,000
Unamortized compensation expense $ 6,151,000    
Expense recognition over weighted average period (in years) 2 years 9 months 15 days    
Restricted stock awards [Roll Forward]      
Beginning balance (in shares) 285,019    
Granted (at market price) (in shares) 95,479    
Vested (in shares) 79,224 95,420 147,619
Forfeited (in shares) 3,750    
Ending balance (in shares) 297,524 285,019  
Restricted stock awards weighted average grant date fair value [Abstract]      
Beginning balance (in dollars per share) $ 64.33    
Granted (at market price) (in dollars per share) 115.38    
Vested (in dollars per share) 52.73    
Forfeited (in dollars per share) 80.69    
Ending balance (in dollars per share) $ 83.36 $ 64.33  
Vested (in shares) 79,224 95,420 147,619
Fair value of restricted stock awards vested $ 9,260,000 $ 7,960,000 $ 10,182,000
ESPP Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares purchased under the ESPP (in shares) 16,610 8,649 22,555
Grant Date Fair Value of most recent ESPP Purchase (per share) $ 17.30    
Grant Date Fair Value Assumptions [Abstract]      
Share-based compensation $ 355,000 $ 173,000 $ 342,000
LTIP 2007 [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award expiration period 10 years    
Number of shares allocated for issuance (in shares) 9,315,000    
Shares available for grant 4,615,148    
Employee Stock Purchase Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares allocated for issuance (in shares) 956,250    
Discounted percentage rate offered under the employee stock purchase plan (in hundredths) 85.00%    
Minimum [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award performance period, extension period 1 year    
Maximum [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award performance period, extension period 2 years    
Split Vesting [Member] | Minimum [Member] | Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Split Vesting [Member] | Minimum [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Split Vesting [Member] | Maximum [Member] | Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 5 years    
Split Vesting [Member] | Maximum [Member] | Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 5 years