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Equity Incentive Plan (Details) - 2010 Plan
$ / shares in Units, $ in Millions
6 Months Ended
Jun. 30, 2020
USD ($)
$ / shares
shares
Jun. 30, 2019
Equity Incentive Plan    
Shares of common stock reserved for issuance 6,000,000  
Shares of common stock that were available for issuance 2,470,665  
Stock Options    
Number of Shares Subject to Option    
Options outstanding at the beginning of the period (in shares) 31,746  
Exercised (in shares) (11,882)  
Options outstanding at the end of the period (in shares) 19,864  
Weighted Average Exercise Price    
Options outstanding at the beginning of the period (in dollars per share) | $ / shares $ 19.57  
Exercised (in dollars per share) | $ / shares 16.77  
Options outstanding at the end of the period (in dollars per share) | $ / shares $ 21.25  
Restricted Awards    
Compensation cost not yet recognized    
Unearned compensation | $ $ 29.1  
Restricted Awards | Weighted Average    
Compensation cost not yet recognized    
Vesting period 2 years 8 months 12 days  
Restricted awards    
Restricted Awards and PSA's    
Unvested balance at the beginning of the period (in shares) 292,373  
Granted (in shares) 155,263  
Forfeited (in shares) (9,889)  
Vested (in shares) (122,230)  
Unvested balance at the end of the period (in shares) 315,517  
Weighted Average Fair Value at Grant    
Unvested balance at the beginning of the period (in dollars per share) | $ / shares $ 96.44  
Granted (in dollars per share) | $ / shares 110.69  
Forfeited (in dollars per share) | $ / shares 101.17  
Vested (in dollars per share) | $ / shares 91.73  
Unvested balance at the end of the period (in dollars per share) | $ / shares $ 105.13  
PSAs    
Compensation cost not yet recognized    
Unearned compensation | $ $ 8.3  
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) 24.00% 24.09%
Risk-free rate (as a percent) 0.56% 2.48%
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 $ 107.84  
Granted (in dollars per share) | $ / shares 125.02  
Forfeited (in dollars per share) | $ / shares 107.63  
Vested (in dollars per share) | $ / shares 105.55  
Unvested balance at the end of the period (in dollars per share) | $ / shares $ 113.96  
Compensation cost not yet recognized    
Vesting period 2 years 2 months 12 days  
PSAs | Minimum    
Restricted Awards and PSA's    
Unvested balance at the beginning of the period (in shares) 34,624  
Granted (in shares) 10,210  
Performance adjustment 38,047  
Forfeited (in shares) (1,049)  
Vested (in shares) (32,524)  
Unvested balance at the end of the period (in shares) 49,308  
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) 171,351  
Granted (in shares) 71,488  
Performance adjustment (7,904)  
Forfeited (in shares) (4,878)  
Vested (in shares) (32,524)  
Unvested balance at the end of the period (in shares) 197,533  
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) 102,989  
Granted (in shares) 40,852  
Performance adjustment 15,073  
Forfeited (in shares) (2,964)  
Vested (in shares) (32,524)  
Unvested balance at the end of the period (in shares) 123,426