COMMITMENTS AND CONTINGENCIES - Part 1 (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2017 USD ($)
Operating Leased Assets [Line Items]
Operating Leases, Rent Expense	$ 8,200	$ 8,700	$ 8,600
Operating Leases, Future Minimum Payments Due [Abstract]
2017	6,300
2018	5,100
2019	4,500
2020	4,300
2021	4,200
Thereafter	3,800
Total	28,200
Regulatory Assets, Noncurrent	$ 437,479	405,189
Number Of Sites Soil Remediation Is Completed Or Near Completion	12
MGP Costs [Member]
Operating Leases, Future Minimum Payments Due [Abstract]
Regulatory Assets, Noncurrent	$ 7,724	$ 6,104		$ 5,900