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Equity Incentive Award Plan (Details) - 2010 Plan - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Equity Incentive Plan    
Shares of common stock reserved for issuance 6,000,000  
Shares of common stock that were available for issuance 2,855,456  
Stock Options    
Number of Shares Subject to Option    
Options outstanding at the beginning of the period (in shares) 67,789  
Exercised (in shares) (12,881)  
Options outstanding at the end of the period (in shares) 54,908  
Weighted Average Exercise Price    
Options outstanding at the beginning of the period (in dollars per share) $ 19.12  
Exercised (in dollars per share) 17.04  
Options outstanding at the end of the period (in dollars per share) $ 19.60  
Restricted Awards    
Restricted Awards and PSA's    
Unvested balance at the beginning of the period (in shares) 289,843  
Granted (in shares) 153,219  
Forfeited (in shares) (35,271)  
Vested (in shares) (122,500)  
Unvested balance at the end of the period (in shares) 285,291  
Weighted Average Fair Value at Grant    
Unvested balance at the beginning of the period (in dollars per share) $ 70.71  
Granted (in dollars per share) 96.95  
Forfeited (in dollars per share) 83.56  
Vested (in dollars per share) 64.39  
Unvested balance at the end of the period (in dollars per share) $ 85.93  
Compensation cost not yet recognized    
Unearned compensation $ 19.3  
Restricted Awards | Weighted Average    
Compensation cost not yet recognized    
Vesting period 2 years 6 months  
PSAs    
Compensation cost not yet recognized    
Unearned compensation $ 5.4  
Assumptions used to value the stock options granted    
Expected term 2 years 9 months 26 days 2 years 9 months 22 days
Expected volatility (as a percent) 24.15% 23.33%
Risk-free rate (as a percent) 2.24% 1.60%
PSAs | Weighted Average    
Weighted Average Fair Value at Grant    
Unvested balance at the beginning of the period (in dollars per share) $ 84.63  
Granted (in dollars per share) 101.03  
Vested (in dollars per share) 61.22  
Unvested balance at the end of the period (in dollars per share) $ 96.58  
Compensation cost not yet recognized    
Vesting period 2 years 1 month 6 days  
PSAs | Minimum    
Restricted Awards and PSA's    
Unvested balance at the beginning of the period (in shares) 38,694  
Granted (in shares) 10,729  
Performance adjustment 33,403  
Vested (in shares) (32,003)  
Unvested balance at the end of the period (in shares) 50,823  
Compensation cost not yet recognized    
Percentage of target number of shares for PSAs granted in years 2015, 2016 and 2017 25.00%  
Assumptions used to value the stock options granted    
Performance period 1 year  
PSAs | Maximum    
Restricted Awards and PSA's    
Unvested balance at the beginning of the period (in shares) 130,843  
Granted (in shares) 75,104  
Vested (in shares) (32,003)  
Unvested balance at the end of the period (in shares) 173,944  
Compensation cost not yet recognized    
Percentage of target number of shares for PSAs granted in years 2015, 2016 and 2017 175.00%  
Assumptions used to value the stock options granted    
Performance period 3 years  
PSAs | Target    
Restricted Awards and PSA's    
Unvested balance at the beginning of the period (in shares) 84,769  
Granted (in shares) 42,917  
Performance adjustment 16,702  
Vested (in shares) (32,003)  
Unvested balance at the end of the period (in shares) 112,385