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UNCONSOLIDATED AFFILIATES (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings from investments $ 34,118 $ 36,570 $ 77,599 $ 76,757  
Accounts Payable, Related Parties, Current 13,500   13,500   $ 14,700
Income Statement [Abstract]          
Revenues 153,205 151,771 320,251 310,679  
Operating expenses 77,704 68,120 147,504 136,521  
Net income 68,916 77,913 159,813 162,393  
Distributions paid to us (a) 99,527 48,424 158,990 98,240  
Unconsolidated Affiliates [Member]          
Schedule of Equity Method Investments [Line Items]          
Related Party Transaction, Expenses from Transactions with Related Parties 40,500 38,200 82,300 75,700  
Northern Border Pipeline          
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings from investments 13,689 15,240 34,491 32,377  
Income Statement [Abstract]          
Distributions paid to us (a) 50,000        
Overland Pass Pipeline          
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings from investments 15,465 16,246 32,859 32,633  
Roadrunner          
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings from investments 6,077 5,542 12,415 10,500  
Other Unconsolidated Affiliates          
Schedule of Equity Method Investments [Line Items]          
Equity in net earnings from investments $ (1,113) $ (458) $ (2,166) $ 1,247