REVENUE Disaggregated Revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Revenue	$ 443,979
Residential
Disaggregation of Revenue [Line Items]
Revenue	155,878
Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue	282,234
OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Revenue	5,204
Other
Disaggregation of Revenue [Line Items]
Revenue	663
Gas
Disaggregation of Revenue [Line Items]
Revenue	396,452
Electric
Disaggregation of Revenue [Line Items]
Revenue	28,378
Solar
Disaggregation of Revenue [Line Items]
Revenue	9,308
CHP
Disaggregation of Revenue [Line Items]
Revenue	7,853
Landfills
Disaggregation of Revenue [Line Items]
Revenue	1,468
Other
Disaggregation of Revenue [Line Items]
Revenue	520
Gas Utility Operations
Disaggregation of Revenue [Line Items]
Revenue	193,934
Gas Utility Operations | Residential
Disaggregation of Revenue [Line Items]
Revenue	147,262
Gas Utility Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue	40,805
Gas Utility Operations | OSS & Capacity Release
Disaggregation of Revenue [Line Items]
Revenue	5,204
Gas Utility Operations | Other
Disaggregation of Revenue [Line Items]
Revenue	663
Gas Utility Operations | Gas
Disaggregation of Revenue [Line Items]
Revenue	193,934
Wholesale Energy Operations
Disaggregation of Revenue [Line Items]
Revenue	173,846
Wholesale Energy Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue	173,846
Wholesale Energy Operations | Gas
Disaggregation of Revenue [Line Items]
Revenue	173,846
Retail Gas and Other Operations
Disaggregation of Revenue [Line Items]
Revenue	33,246
Retail Gas and Other Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue	33,246
Retail Gas and Other Operations | Gas
Disaggregation of Revenue [Line Items]
Revenue	33,246
On-Site Energy Production
Disaggregation of Revenue [Line Items]
Revenue	21,157
On-Site Energy Production | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue	21,157
On-Site Energy Production | Solar
Disaggregation of Revenue [Line Items]
Revenue	11,836
On-Site Energy Production | CHP
Disaggregation of Revenue [Line Items]
Revenue	7,853
On-Site Energy Production | Landfills
Disaggregation of Revenue [Line Items]
Revenue	1,468
Corporate Services and Intersegment
Disaggregation of Revenue [Line Items]
Revenue	(8,770)
Corporate Services and Intersegment | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue	(8,770)
Corporate Services and Intersegment | Gas
Disaggregation of Revenue [Line Items]
Revenue	(4,574)
Corporate Services and Intersegment | Electric
Disaggregation of Revenue [Line Items]
Revenue	(1,668)
Corporate Services and Intersegment | Solar
Disaggregation of Revenue [Line Items]
Revenue	(2,528)
Retail Electric Operations
Disaggregation of Revenue [Line Items]
Revenue	30,046
Retail Electric Operations | Residential
Disaggregation of Revenue [Line Items]
Revenue	8,096
Retail Electric Operations | Commercial & Industrial
Disaggregation of Revenue [Line Items]
Revenue	21,950
Retail Electric Operations | Electric
Disaggregation of Revenue [Line Items]
Revenue	30,046
Appliance Service Operations
Disaggregation of Revenue [Line Items]
Revenue	520
Appliance Service Operations | Residential
Disaggregation of Revenue [Line Items]
Revenue	520
Appliance Service Operations | Other
Disaggregation of Revenue [Line Items]
Revenue	$ 520