BUSINESS COMBINATION - EGT and ELK Acquisition (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Aug. 31, 2019	Dec. 31, 2018	Jul. 01, 2018
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Property, Plant and Equipment				$ 1,202,435
Accounts Receivable				45,875
Provision for Uncollectibles				(6,579)
Natural Gas in Storage				12,204
Materials and Supplies				345
Other Prepayments and Current Assets				200
Deferred Income Taxes				39,470
Regulatory Assets				136,212
Goodwill	$ 705,707		$ 734,607	700,286
Total assets acquired				2,130,448
Accounts Payable				13,089
Other Current Liabilities				9,185
Environmental Remediation Costs - Current				7,100
Pension and Other Postretirement Benefits				3,213
Environmental Remediation Costs - Non Current				66,165
Regulatory Liabilities				192,811
Asset Retirement Obligation				113,093
Other				1,107
Total liabilities assumed				405,763
Total net assets acquired				$ 1,724,685
AEP
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Accounts Receivable		$ 116
Goodwill		1,843
Total assets acquired		4,455
Accounts Payable		11
Other Current Liabilities		449
Total liabilities assumed		460
Total net assets acquired		$ 3,995