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Summary of Significant Accounting Policies (Regulatory Rate Filings) (Details)
 6 Months Ended 12 Months Ended
Feb. 05, 2016
USD ($)
$ / Ccf
Dec. 15, 2015
USD ($)
$ / Ccf
Dec. 03, 2015
USD ($)
Jun. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Aug. 21, 2015
USD ($)
May. 29, 2015
USD ($)
Apr. 23, 2015
USD ($)
Mar. 26, 2015
USD ($)
Jan. 05, 2015
USD ($)
Public Utilities General Disclosures [Line Items]                        
Regulatory Balancing and Memorandum Accounts, Net Under-Collected         $ 1,081,000              
Authorized Regulatory Surcharge, Gross       $ 1,937,000                
Requested rate increase for 2016                       $ 34,928,000
Requested rate increase for 2016 as percentage of total revenue at time of request                       12.22%
Requested rate increase for 2017                       $ 9,954,000
Requested rate increase for 2017 as percentage of total revenue at time of request                       3.11%
Requested rate increase for 2018                       $ 17,567,000
Requested rate increase for 2018 as percentage of total revenue at time of request                       5.36%
Memorandum Account, Recovery Request                     $ 9,567,000 $ 976,000
Recommended rate increase for 2016                   $ 23,468,000    
Recommended rate increase for 2016 as percentage of total revenue at time of request                   8.54%    
Recommended rate increase for 2017                   $ 13,803,000    
Recommended rate increase for 2017 as percentage of total revenue at time of request                   4.42%    
Recommended rate increase for 2018                   $ 16,261,000    
Recommended rate increase for 2018 as percentage of total revenue at time of request                   4.95%    
Regulatory Rate Filings, Requested Issuance of Debt and Equity Securities               $ 150,000,000        
Regulatory rate filings, requested rate increase, authorized revenue               $ 275,000 $ 18,357,000      
Total increase requested, percentage               0.09% 6.44%      
Requested Regulatory Surcharge, Per CCF | $ / Ccf   0.0492                    
Balancing Account, Recovery Request   $ 2,599,000                   $ 4,752,000
Balancing Account, Uncollected   $ 590,000                    
Balancing and Memorandum Account, Surcharge Offset         0 $ 0 $ 0          
Regulatory Assets and Liabilities [Abstract]                        
Net Regulatory Assets         168,563,000 174,863,000            
Current regulatory assets, net         16,542,000 16,853,000            
Total regulatory assets, net, less current portion         $ 152,021,000 158,010,000            
Regulatory Balancing and Memorandum Accounts Review, Authorized Revenue, Threshold Percentage         2.00%              
Income tax temporary differences [Member]                        
Regulatory Assets and Liabilities [Abstract]                        
Regulatory assets         $ 8,184,000 6,731,000            
Postretirement pensions and other medical benefits [Member]                        
Regulatory Assets and Liabilities [Abstract]                        
Regulatory assets         109,168,000 115,494,000            
Balancing and Memorandum Accounts [Member]                        
Regulatory Assets and Liabilities [Abstract]                        
Regulatory assets         45,206,000 47,061,000            
Other Regulatory Assets [Member]                        
Regulatory Assets and Liabilities [Abstract]                        
Regulatory assets         6,005,000 $ 5,577,000            
2014 WCMA [Member]                        
Public Utilities General Disclosures [Line Items]                        
Memorandum Account, Authorized Recovery     $ 4,259,000                  
Authorized Regulatory Surcharge, Per CCF     $ 0.08                  
WCMA Reserve         1,278,000              
Authorized Regulatory Surcharge, Net         2,981,000              
2015 WCMA [Member]                        
Public Utilities General Disclosures [Line Items]                        
Memorandum Account, Recovery Request         19,854,000              
WCMA Reserve         2,343,000              
Balancing and Memorandum Account, Surcharge Offset         12,139,000              
Memorandum Account, Recorded Net         $ 5,372,000              
Subsequent Event [Member]                        
Public Utilities General Disclosures [Line Items]                        
Memorandum Account, Authorized Recovery $ 7,668,000                      
Requested Regulatory Surcharge, Per CCF | $ / Ccf 0.1441