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Stock-Based Compensation Plans 2 (Details) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Number of Options      
Common Shares issued as a result of the exercise of stock options 32,142 52,936 12,546
Options outstanding at the end of the period (in shares) 35,560    
Additional disclosure      
Cash proceeds from the exercise of stock options $ 546,000 $ 909,000 $ 235,000
Tax benefit for the tax deduction from awards exercised 0 $ 0 $ (581,000)
Unrecognized compensation cost $ 300,000    
Maximum      
Additional disclosure      
Percentage of target amount of performance shares 200.00%    
Minimum      
Additional disclosure      
Percentage of target amount of performance shares 0.00%    
Weighted Average [Member]      
Additional disclosure      
Expected recognition period for unrecognized compensation cost 1 year 9 months 63 days    
Stock options      
Number of Options      
Options Outstanding at the beginning of the period (in shares) 69,202    
Granted (in shares) 0 0 0
Common Shares issued as a result of the exercise of stock options 32,142    
Forfeited or expired (in shares) (1,500)    
Options outstanding at the end of the period (in shares) 35,560 69,202  
Options exercisable at the end of the period (in shares) 35,560    
Weighted Average Exercise Price      
Options outstanding at the beginning of the period (in dollars per share) $ 16.87    
Exercised (in dollars per share) 16.98    
Forfeited or expired (in dollars per share) 17.06    
Options outstanding at the end of the period (in dollars per share) 16.76 $ 16.87  
Options exercisable at the end of the period (in dollars per share) $ 16.76    
Additional disclosure      
Weighted Average Remaining Contractual Term, Options outstanding 9 months 88 days    
Weighted Average Remaining Contractual Term, Options Exercisable 9 months 88 days    
Aggregate Intrinsic Value, Options outstanding $ 1,788,028    
Aggregate Intrinsic Value, Options Exercisable 1,788,028    
Total intrinsic value of options exercised 1,367,000 $ 1,718,000 $ 308,000
Tax benefit for the tax deduction from awards exercised (1,600,000) $ (1,024,000) (581,000)
Restricted Stock Units      
Additional disclosure      
Unrecognized compensation cost related to performance awards $ 611,000    
Number of Restricted/Performance Share Units      
Restricted share units at the beginning of the period (in shares) 107,287    
Granted (in shares) 33,655    
Vested (in shares) (37,509)    
Forfeited (in shares) (1,198)    
Restricted share units at the end of the period (in shares) 102,235 107,287  
Weighted Average Grant-Date Value      
Restricted share units at the beginning of the period (in dollars per share) $ 32.75    
Granted (in dollars per share) 55.45    
Vested (in dollars per share) 47.29    
Forfeited (in dollars per share) 46.94    
Restricted share units at the end of the period (in dollars per share) $ 34.73 $ 32.75  
Restricted Stock Units | Weighted Average [Member]      
Additional disclosure      
Expected recognition period for unrecognized compensation cost 1 year 5 months 38 days    
Performance awards      
Additional disclosure      
Vesting period 3 years    
Period to meet the performance goals 3 years    
Number of Restricted/Performance Share Units      
Restricted share units at the beginning of the period (in shares) 97,879    
Granted (in shares) 25,195    
Performance criteria adjustment (in shares) 3,803    
Vested (in shares) (31,216)    
Restricted share units at the end of the period (in shares) 95,661 97,879  
Weighted Average Grant-Date Value      
Restricted share units at the beginning of the period (in dollars per share) $ 41.49    
Granted (in dollars per share) 55.88    
Performance criteria adjustment (in dollars per share) 43.98    
Vested (in dollars per share) 41.55    
Restricted share units at the end of the period (in dollars per share) $ 45.36 $ 41.49  
GSWC      
Additional disclosure      
Tax benefit for the tax deduction from awards exercised $ 0 $ 0 (501,000)
GSWC | Stock options      
Additional disclosure      
Tax benefit for the tax deduction from awards exercised $ (1,600,000) $ (1,012,000) $ (501,000)