UNCONSOLIDATED AFFILIATES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investments	$ 1,003,156	$ 958,807
Equity Method Goodwill	38,800	40,100
Equity in net earnings from investments	159,278	139,690	$ 125,300
Impairment of equity investments	(4,270)	0	(180,583)
Balance Sheet [Abstract]
Current assets	151,907	143,317
Property, plant and equipment, net	2,490,692	2,579,607
Other noncurrent assets	14,793	20,784
Current liabilities	70,434	77,388
Long-term debt	479,050	649,539
Other noncurrent liabilities	53,830	69,265
Accumulated other comprehensive loss	(9,946)	(7,450)
Owners’ equity	2,064,024	1,954,966
Income Statement [Abstract]
Operating revenues	639,102	578,542	524,496
Operating expenses	277,121	260,753	304,930
Net income	347,692	293,921	200,064
Distributions paid to us	196,114	196,717	155,918
Accounts Payable, Related Parties, Current	13,600	11,100
Payments to Acquire Other Investments	87,861	68,275	27,540
Unconsolidated Affiliates [Member]
Income Statement [Abstract]
Related Party Transaction, Expenses from Transactions with Related Party	156,100	$ 140,300	104,700
Northern Border Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Net ownership percentage		50.00%		50.00%	50.00%
Investments	396,800	$ 328,456
Equity in net earnings from investments	68,153	$ 69,990	66,941
Income Statement [Abstract]
Payments to Acquire Equity Method Investments	83,000
Overland Pass Pipeline Company [Member]
Schedule of Equity Method Investments [Line Items]
Net ownership percentage		50.00%		50.00%
Investments	436,111	$ 444,138
Equity in net earnings from investments	60,067	$ 53,984	37,783
Roadrunner Gas Transmission [Member]
Schedule of Equity Method Investments [Line Items]
Net ownership percentage		50.00%		50.00%
Investments	93,048	$ 94,548
Equity in net earnings from investments	19,150	4,445	1,800
Income Statement [Abstract]
Payments to Acquire Equity Method Investments	4,000	65,000
Other Unconsolidated Affiliates [Member]
Schedule of Equity Method Investments [Line Items]
Investments	77,197	91,665
Equity in net earnings from investments	$ 11,908	$ 11,271	$ 18,776