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UNCONSOLIDATED AFFILIATES (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings (loss) from investments $ 39,313 $ 40,058 $ 116,070 $ 118,985  
Impairment of equity investments 0 (4,270) 0 (4,270)  
Accounts Payable, Related Parties, Current 12,600   12,600   $ 13,600
Income Statement [Abstract]          
Operating revenues 160,962 163,627 471,641 475,510  
Operating expenses 69,004 69,740 205,525 206,141  
Net income 85,361 87,330 247,754 260,533  
Distributions paid to us 47,197 49,414 145,437 146,094  
Unconsolidated Affiliates [Member]          
Schedule of Equity Method Investments [Line Items]          
Related Party Transaction, Expenses from Transactions with Related Parties 37,500 39,900 113,200 116,000  
Northern Border Pipeline          
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings (loss) from investments 16,486 16,440 48,863 50,879  
Income Statement [Abstract]          
Payments to Acquire Equity Method Investments     0 83,000  
Overland Pass Pipeline Company          
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings (loss) from investments 16,081 15,793 48,714 44,243  
Roadrunner Gas Transmission          
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings (loss) from investments 6,303 4,898 16,803 14,192  
Income Statement [Abstract]          
Payments to Acquire Equity Method Investments     500 4,000  
Other Unconsolidated Affiliates          
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings (loss) from investments $ 443 $ 2,927 $ 1,690 $ 9,671