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Revenue Disaggregation of Revenue (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax $ 265,008  
Revenues 264,712 $ 297,323
Local Gas Distribution [Member] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 258,229  
Natural Gas Storage [Domain] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 3,788  
Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 1,445  
Appliance Center [Member] | Transferred at Point in Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 1,546  
Alternative Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Revenues (372)  
Leasing Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Leasing revenue 76  
Utility Segment [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 258,229  
Revenues 257,933  
Utility Segment [Member] | Local Gas Distribution [Member] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 258,229  
Utility Segment [Member] | Natural Gas Storage [Domain] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 0  
Utility Segment [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 0  
Utility Segment [Member] | Appliance Center [Member] | Transferred at Point in Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Including Assessed Tax 0  
Utility Segment [Member] | Alternative Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Revenues (372)  
Utility Segment [Member] | Leasing Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Leasing revenue 76  
Gas Storage Segment [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 5,233  
Revenues 5,233  
Gas Storage Segment [Member] | Local Gas Distribution [Member] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 0  
Gas Storage Segment [Member] | Natural Gas Storage [Domain] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 3,788  
Gas Storage Segment [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 1,445  
Gas Storage Segment [Member] | Appliance Center [Member] | Transferred at Point in Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 0  
Gas Storage Segment [Member] | Alternative Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Revenues 0  
Gas Storage Segment [Member] | Leasing Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Leasing revenue 0  
Other Segments [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 1,546  
Revenues 1,546  
Other Segments [Member] | Local Gas Distribution [Member] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 0  
Other Segments [Member] | Natural Gas Storage [Domain] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 0  
Other Segments [Member] | Asset Management Revenue, Net of Regulatory Sharing [Member] | Transferred over Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 0  
Other Segments [Member] | Appliance Center [Member] | Transferred at Point in Time [Member]    
Disaggregation of Revenue [Line Items]    
Revenue from Contract with Customer, Excluding Assessed Tax 1,546  
Other Segments [Member] | Alternative Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Revenues 0  
Other Segments [Member] | Leasing Revenue [Member]    
Disaggregation of Revenue [Line Items]    
Leasing revenue $ 0