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Stock-Based Compensation (Details) - USD ($)
12 Months Ended 24 Months Ended 36 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense $ (924,000) $ (1,012,000) $ (861,000)        
Allocated Share-based Compensation Expense, Net of Tax 1,446,000 1,661,000 1,448,000        
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount $ 554,000 $ 661,000 $ 597,000        
Long Term Incentive Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 850,000       850,000    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant, Any Award Type 173,279       173,279    
Share-based Compensation Arrangement By Share Based Payment Award, Number of Shares Available For Grant Stock Options 250,000       250,000    
Option Shares              
Balance outstanding, end of period 0       0    
Number of RSUs [Roll Forward]              
Nonvested, end of period 0       0    
Long Term Incentive Plan [Member] | Performance Shares [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share Based Compensation Arrangement by Share Based Payment Award Performance Period 3 years            
Share Based Compensation Award Target Share 103,875       103,875    
Unrecognized compensation expense $ 2,200,000       $ 2,200,000    
Unrecognized compensation cost, period for recognition through 2018.            
Allocated Share-based Compensation Expense $ 1,308,000            
Weighted Average Price Per LTIP              
Nonvested, beginning of period $ 49.09            
Granted 50.15            
Vested 51.80            
Nonvested, end of period $ 50.83 $ 49.09   $ 49.09 $ 50.83 $ 49.09  
Long Term Incentive Plan [Member] | Performance Shares [Member] | Maximum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share Based Compensation Award Target Share 207,750       207,750    
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2012-2014 Grant              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted [1]     8,621        
Allocated Share-based Compensation Expense     $ 582,000 [2]       $ 1,821,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2014-2016 Grant              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted [1],[3] 27,887            
Share Based Compensation Award Target Share 39,725       39,725    
Allocated Share-based Compensation Expense [3] $ 168,000 [2]       $ 1,418,000    
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2014-2016 Grant | Maximum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share Based Compensation Award Target Share 79,450       79,450    
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2013 Award [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted [1]   8,914          
Allocated Share-based Compensation Expense   $ 312,000 [2]       $ 1,240,000  
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2015-2017 Grant              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share Based Compensation Award Target Share 36,200       36,200    
Allocated Share-based Compensation Expense $ 662,000     $ 1,515,000      
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2015-2017 Grant | Maximum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share Based Compensation Award Target Share 72,400       72,400    
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2016 Award [Member] [Domain]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share Based Compensation Award Target Share 27,950       27,950    
Allocated Share-based Compensation Expense $ 478,000            
Long Term Incentive Plan [Member] | Performance Shares [Member] | A 2016 Award [Member] [Domain] | Maximum [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Share Based Compensation Award Target Share 55,900       55,900    
Long Term Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Unrecognized compensation expense $ 2,900,000       $ 2,900,000    
Unrecognized compensation cost, period for recognition through 2022            
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 4 years            
Allocated Share-based Compensation Expense $ 1,500,000 $ 1,300,000          
Number of RSUs [Roll Forward]              
Nonvested, beginning of period 88,587 70,794 44,567 44,567 44,567    
Granted 40,271 37,264 38,765        
Vested (29,488) (19,003) (12,060)        
Forfeit (9,397) (468) (478)        
Nonvested, end of period 89,973 88,587 70,794 88,587 89,973 88,587 70,794
Weighted Average Price Per LTIP              
Nonvested, beginning of period $ 44.78 $ 44.00 $ 46.27 $ 46.27 $ 46.27    
Granted 54.36 46.29 42.19        
Vested 45.56 44.81 46.52        
Forfeited 44.59 44.99 45.47        
Nonvested, end of period $ 48.85 $ 44.78 $ 44.00 $ 44.78 $ 48.85 $ 44.78 $ 44.00
Restated Stock Option Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options $ 7,500,000            
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period 10 years 7 days            
Share Based Compensation Arrangement By Share Based Payment Award Exchange Period 6 months            
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value   $ 200,000          
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term 3 years 22 days            
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term 3 years 22 days            
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense $ (393,000)            
Option Shares              
Balance outstanding, beginning of period 352,688 416,088 492,150 492,150 492,150    
Exercised (172,525) (62,900) (69,662)        
Forfeited 0 (500) (6,400)        
Balance outstanding, end of period 180,163 352,688 416,088 352,688 180,163 352,688 416,088
Other Option disclosures              
Balance exercisable, end of period 180,163       180,163    
Weighted-average price per share exercisable, end of period $ 44.38       $ 44.38    
Intrinsic value balance, end of period, Exercisable $ 2,800,000       $ 2,800,000    
Weighted-Average Price Per Share              
Balance outstanding, beginning of period $ 44.00 $ 43.40 $ 42.89 $ 42.89 $ 42.89    
Exercised 43.61 39.96 39.82        
Forfeited 0.00 45.74 43.59        
Balance outstanding, end of period $ 44.38 $ 44.00 $ 43.40 $ 44.00 $ 44.38 $ 44.00 $ 43.40
Intrinsic Value balance, beginning of period $ 2,300,000 $ 2,700,000 $ 600,000 $ 600,000 $ 600,000    
Intrinsic value, Exercised 2,000,000 500,000 500,000        
Intrinsic Value balance, end of period $ 2,800,000 2,300,000 2,700,000 $ 2,300,000 $ 2,800,000 $ 2,300,000 $ 2,700,000
Employee Stock Purchase Plan [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Percent Of Offering Date Price Employees Are Allowed To Purchase Stock 85.00%            
Maximum Value Of Stock Employees Are Allowed To Purchase $ 21,248            
Operations and Maintenance Expense [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Allocated Share-based Compensation Expense $ 2,370,000 $ 2,673,000 $ 2,309,000        
[1] In addition to common stock shares, a participant also receives a dividend equivalent cash payment equal to the number of shares of common stock received on the award payout multiplied by the aggregate cash dividends paid per share during the performance period.
[2] Amount represents the expense recognized in the third year of the vesting period noted above.
[3] This represents the estimated number of shares to be awarded as of December 31, 2016 as certain performance share measures had been achieved. Amounts are subject to change with final payout amounts authorized by the Board of Directors in February 2017.