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FAIR VALUE MEASUREMENTS - Part 1 (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash held - offsetting derivative net asset positions under master-netting arrangements $ 0 $ 0
Cash posted - total 67,800 49,700
Cash posted - offsetting derivative net liability positions under master-netting arrangements 46,200 29,500
Cash posted - remaining in excess of derivative net liability positions included in consolidated Balance Sheets 21,600 20,200
Long-term debt, Fair Value 9,300,000 9,300,000
Long-term debt 8,800,000 8,500,000
Fair Value, Measurements, Recurring [Member]    
Derivative assets    
Financial contracts 0 0
Interest-rate contracts 50,509 49,960
Total derivative assets 50,509 49,960
Derivative assets netting (54,129) (24,455)
Derivative liabilities    
Financial contracts 0 (32)
Physical contracts (2,010) (4,781)
Total derivative liabilities (2,010) (4,813)
Derivative liabilities netting 100,350 53,936
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]    
Derivative assets    
Financial contracts 54,129 24,455
Interest-rate contracts 50,509 49,960
Total derivative assets 104,638 74,415
Derivative liabilities    
Financial contracts (100,350) (53,968)
Physical contracts (2,010) (4,781)
Total derivative liabilities (102,360) (58,749)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Derivative assets    
Financial contracts 183 4,252
Interest-rate contracts 0 0
Total derivative assets 183 4,252
Derivative liabilities    
Financial contracts (17,337) (5,708)
Physical contracts 0 0
Total derivative liabilities (17,337) (5,708)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Derivative assets    
Financial contracts 0 0
Interest-rate contracts 50,509 49,960
Total derivative assets 50,509 49,960
Derivative liabilities    
Financial contracts 0 0
Physical contracts 0 0
Total derivative liabilities 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Derivative assets    
Financial contracts 53,946 20,203
Interest-rate contracts 0 0
Total derivative assets 53,946 20,203
Derivative liabilities    
Financial contracts (83,013) (48,260)
Physical contracts (2,010) (4,781)
Total derivative liabilities $ (85,023) $ (53,041)