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Stock-based Compensation (Details) - Equity Incentive Plan - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Compensation expense        
Recorded compensation costs for the RSAs and RSUs     $ 2.0 $ 1.5
Restricted Stock Awards (RSAs)        
Stock-based Compensation        
Weighted average grant date fair value (in dollars per share)     $ 24.29 $ 23.61
Restricted Stock Awards (RSAs) | Officers and Directors        
Stock-based Compensation        
Awards granted (in shares)   2 61 60
Awards cancelled (in shares) 3 0 16 11
Restricted Stock Awards (RSAs) | Employees        
Stock-based Compensation        
Vesting period     36 months  
Restricted Stock Awards (RSAs) | Director        
Stock-based Compensation        
Vesting period     12 months  
Restricted Stock Unit Award (RSUs) | Officer        
Stock-based Compensation        
Awards granted (in shares)     39 37
Expiration period of award     3 years 3 years
Weighted average grant date fair value (in dollars per share)     $ 24.28 $ 23.61
Restricted Stock Unit Award (RSUs) | Officer | Minimum        
Stock-based Compensation        
Options vested on anniversary date (as a percent)     0.00%  
Restricted Stock Unit Award (RSUs) | Officer | Maximum        
Stock-based Compensation        
Options vested on anniversary date (as a percent)     200.00%