UNCONSOLIDATED AFFILIATES (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Equity in net earnings from investments	$ 37,576	$ 39,313	$ 115,175	$ 116,070
Accounts Payable, Related Parties, Current	13,800		13,800		$ 14,700
Income Statement [Abstract]
Revenues	151,210	160,962	471,461	471,641
Operating expenses	69,783	69,004	217,287	205,525
Net income	74,341	85,361	234,154	247,754
Distributions paid to us	44,357	47,197	203,347	145,437
Unconsolidated Affiliates [Member]
Schedule of Equity Method Investments [Line Items]
Related Party Transaction, Expenses from Transactions with Related Parties	41,900	37,500	124,200	113,200
Northern Border Pipeline
Schedule of Equity Method Investments [Line Items]
Equity in net earnings from investments	16,102	16,486	50,593	48,863
Income Statement [Abstract]
Distributions paid to us			50,000
Overland Pass Pipeline
Schedule of Equity Method Investments [Line Items]
Equity in net earnings from investments	15,481	16,081	48,340	48,714
Roadrunner
Schedule of Equity Method Investments [Line Items]
Equity in net earnings from investments	7,421	6,303	19,836	16,803
Other Unconsolidated Affiliates
Schedule of Equity Method Investments [Line Items]
Equity in net earnings from investments	$ (1,428)	$ 443	$ (3,594)	$ 1,690