UNCONSOLIDATED AFFILIATES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Net ownership percentage				50.00%
Investments	$ 969,150	$ 1,003,156
Equity Method Goodwill	38,800
Equity in net earnings from investments	158,383	159,278	$ 139,690
Impairment of equity investments	0	(4,270)	0
Balance Sheet [Abstract]
Current assets	158,723	151,907
Property, plant and equipment, net	2,413,662	2,490,692
Other noncurrent assets	16,273	14,793
Current liabilities	83,057	70,434
Long-term debt	480,731	479,050
Other noncurrent liabilities	47,826	53,830
Accumulated other comprehensive loss	2,053	(9,946)
Owners’ equity	1,974,991	2,064,024
Income Statement [Abstract]
Operating revenues	637,762	639,102	578,542
Operating expenses	276,373	277,121	260,753
Net income	337,694	347,692	293,921
Distributions paid to us	197,285	196,114	196,717
Accounts Payable, Related Parties, Current	14,700	13,600
Unconsolidated Affiliates [Member]
Income Statement [Abstract]
Related Party Transaction, Expenses from Transactions with Related Party	153,900	$ 156,100	140,300
Northern Border Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Net ownership percentage		50.00%		50.00%	50.00%
Investments	381,623	$ 396,800
Equity in net earnings from investments	67,854	68,153	69,990
Income Statement [Abstract]
Payments to Acquire Equity Method Investments	0	$ 83,000
Overland Pass Pipeline Company [Member]
Schedule of Equity Method Investments [Line Items]
Net ownership percentage		50.00%		50.00%
Investments	429,295	$ 436,111
Equity in net earnings from investments	65,887	$ 60,067	53,984
Roadrunner Gas Transmission [Member]
Schedule of Equity Method Investments [Line Items]
Net ownership percentage		50.00%		50.00%
Investments	93,857	$ 93,048
Equity in net earnings from investments	22,993	19,150	4,445
Income Statement [Abstract]
Payments to Acquire Equity Method Investments			65,000
Other Unconsolidated Affiliates [Member]
Schedule of Equity Method Investments [Line Items]
Investments	64,375	77,197
Equity in net earnings from investments	$ 1,649	$ 11,908	$ 11,271