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Equity Incentive Plan (Details) - 2010 Plan
$ / shares in Units, $ in Millions
9 Months Ended
Sep. 30, 2019
USD ($)
$ / shares
shares
Sep. 30, 2018
Equity Incentive Plan    
Shares of common stock reserved for issuance 6,000,000  
Shares of common stock that were available for issuance 2,674,453  
Stock Options    
Number of Shares Subject to Option    
Options outstanding at the beginning of the period (in shares) 49,133  
Exercised (in shares) (3,622)  
Options outstanding at the end of the period (in shares) 45,511  
Weighted Average Exercise Price    
Options outstanding at the beginning of the period (in dollars per share) | $ / shares $ 20.06  
Exercised (in dollars per share) | $ / shares 18.10  
Options outstanding at the end of the period (in dollars per share) | $ / shares $ 20.21  
Restricted Awards    
Compensation cost not yet recognized    
Unearned compensation | $ $ 22.1  
Restricted Awards | Weighted Average    
Compensation cost not yet recognized    
Vesting period 2 years 6 months  
Restricted awards    
Restricted Awards and PSA's    
Unvested balance at the beginning of the period (in shares) 284,879  
Granted (in shares) 158,608  
Forfeited (in shares) (19,865)  
Vested (in shares) (129,008)  
Unvested balance at the end of the period (in shares) 294,614  
Weighted Average Fair Value at Grant    
Unvested balance at the beginning of the period (in dollars per share) | $ / shares $ 85.98  
Granted (in dollars per share) | $ / shares 100.32  
Forfeited (in dollars per share) | $ / shares 93.65  
Vested (in dollars per share) | $ / shares 78.81  
Unvested balance at the end of the period (in dollars per share) | $ / shares $ 96.32  
PSAs    
Compensation cost not yet recognized    
Unearned compensation | $ $ 6.5  
Assumptions used to value the stock options granted    
Expected term 2 years 9 months 25 days 2 years 9 months 25 days
Expected volatility (as a percent) 24.09% 24.15%
Risk-free rate (as a percent) 2.48% 2.24%
Performance period 3 years  
PSAs | Weighted Average    
Weighted Average Fair Value at Grant    
Unvested balance at the beginning of the period (in dollars per share) | $ / shares $ 96.58  
Granted (in dollars per share) | $ / shares 114.55  
Performance adjustment (in dollars per share) | $ / shares 103.26  
Vested (in dollars per share) | $ / shares 84.78  
Unvested balance at the end of the period (in dollars per share) | $ / shares $ 107.84  
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) 50,823  
Granted (in shares) 10,926  
Performance adjustment 18,063  
Forfeited (in shares) (1,538)  
Vested (in shares) (43,650)  
Unvested balance at the end of the period (in shares) 34,624  
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) 173,944  
Granted (in shares) 76,480  
Performance adjustment (29,819)  
Forfeited (in shares) (5,604)  
Vested (in shares) (43,650)  
Unvested balance at the end of the period (in shares) 171,351  
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) 112,385  
Granted (in shares) 43,703  
Performance adjustment (5,878)  
Forfeited (in shares) (3,571)  
Vested (in shares) (43,650)  
Unvested balance at the end of the period (in shares) 102,989