SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Business Acquisition (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 30, 2017
Business Acquisition [Line Items]
Payments to acquire productive assets, gross		$ 272,965	$ 279,423	$ 343,883
Unamortized debt issuance costs		17,400	7,600
Interest Charges		$ 54,019	$ 31,449	$ 31,622
Elizabethtown Gas and Elkton Gas
Business Acquisition [Line Items]
Payments to acquire productive assets, gross	$ 1,700,000
Unamortized debt issuance costs	12,900
Interest Charges	2,600
Ticking fees	2,500
Amortization of debt issuance costs	10,400
Acquisition Costs, net of tax (A)	$ 27,400
Elizabethtown Gas and Elkton Gas | Bridge Loan
Business Acquisition [Line Items]
Term of agreement	364 days
Short-term Debt	$ 2,600,000				$ 1,700,000
Operating Expense | Elizabethtown Gas and Elkton Gas
Business Acquisition [Line Items]
Acquisition Costs, net of tax (A)	$ 14,500