COMMITMENTS AND CONTINGENCIES - (Details) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Commitments and Contingencies [Line Items]
Number Of Former Manufactured Gas Sites Where We Own Or Retain Legal Responsibility For Environmental Conditions				12
Number of MGP sites for which a remediation reserve was recorded				1
Deferred MGP Costs, Maximum						$ 15,000
Regulatory assets				$ 391,036	$ 437,479
Number of sites with ongoing groundwater monitoring				8
Environmental Reserve Estimate Range, Low		$ 5,600	$ 4,000
Environmental Reserve Estimate Range, High		$ 7,000	$ 7,000
Accrual for Environmental Loss Contingencies, Revision in Estimates	$ 1,600
Environmental Reserve Estimate, Actual	$ 5,600
Percentage yield of high consequence pipeline areas						30.00%
Number Of Sites Soil Remediation Is Completed Or Near Completion				12
MGP Costs [Member]
Commitments and Contingencies [Line Items]
Regulatory assets				$ 9,709	$ 7,724