SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Management Agreement (Details) - USD ($) $ in Thousands		12 Months Ended
	Jul. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Payments to purchase asset management agreement	$ 11,300	$ 11,339	$ 0	$ 0
Natural Gas in Storage	9,685
Intangible Asset	19,200	16,600
Profit Sharing - Other Liabilities	(17,546)	(17,000)
Total Consideration	$ 11,339
ERIP costs expensed		3,300	$ 2,600	$ 2,300
Expected debt refinancing		500,000
ERIP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
ERIP total costs		13,400
ERIP costs expensed		8,300
ERIP costs capitalized		$ 5,100