Quarterly report pursuant to Section 13 or 15(d)

Share-Based Payments - Stock Option Activity (Details)

v3.8.0.1
Share-Based Payments - Stock Option Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense $ 2,261 $ 1,962
Employee Stock Option [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense 342 363
Tax benefit related to share-based expense 88 134
Unrecognized share-based compensation, net of estimated forfeitures $ 2,589 $ 2,683
Weighted average period over which unrecognized compensation will be recognized (years) 2 years 2 months 11 days  
Employee activity - stock options [Abstract]    
Stock option grants expire (in years) 7 years  
Grants, vesting period (in years) 3 years  
Expected dividend yield (in hundredths) 1.10% 1.30%
Expected stock price volatility 24.90% 28.70%
Weighted Average Risk Free Interest Rate 2.60% 2.00%
Expected life of options (in years) 6 years 11 days 6 years 11 days
Weighted-average fair value of options (dollars per share) $ 15 $ 13
Outstanding, beginning of period (in shares) 440  
Granted (in shares) 86  
Exercised (in shares) 0  
Forfeited (in shares) 0  
Outstanding, end of period (in shares) 526  
Exercisable, end of period (in shares) 313  
Outstanding, beginning of period (in dollars per share) $ 45  
Grants in Period (in dollars per share) 59  
Exercises in Period (in dollars per share) 0  
Forfeited in period (in dollars per share) 0  
Outstanding, end of period (in dollars per share) 47  
Exercisable, end of period (in dollars per share) $ 44  
Aggregate Intrinsic Value Outstanding, end of period $ 3,075  
Aggregate Intrinsic Value Exercisable, end of period $ 2,877  
Weighted-average remaining contractual term Outstanding, end of period (in years) 4 years 6 months 11 days  
Weighted-average remaining contractual term Exercisable, end of period (in years) 3 years 6 months 11 days