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FAIR VALUE MEASUREMENTS - Part 1 (Details) - USD ($)
$ in Thousands
Jun. 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 52,600 49,700
Cash posted - offsetting derivative net liability positions under master-netting arrangements 36,200 29,500
Cash posted - remaining in excess of derivative net liability positions included in consolidated Balance Sheets 16,400 20,200
Long-term debt, Fair Value 8,500,000 9,300,000
Long-term debt 8,000,000 8,500,000
Fair Value, Measurements, Recurring [Member]    
Derivative assets    
Financial contracts 0 0
Interest-rate contracts 29,596 49,960
Total derivative assets 29,596 49,960
Derivative assets netting (24,121) (24,455)
Derivative liabilities    
Financial contracts 0 (32)
Physical contracts (1,854) (4,781)
Interest-rate contracts (5,290)  
Total derivative liabilities (7,144) (4,813)
Derivative liabilities netting 60,323 53,936
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]    
Derivative assets    
Financial contracts 24,121 24,455
Interest-rate contracts 29,596 49,960
Total derivative assets 53,717 74,415
Derivative liabilities    
Financial contracts (60,323) (53,968)
Physical contracts (1,854) (4,781)
Interest-rate contracts (5,290)  
Total derivative liabilities (67,467) (58,749)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Derivative assets    
Financial contracts 623 4,252
Interest-rate contracts 0 0
Total derivative assets 623 4,252
Derivative liabilities    
Financial contracts (15,178) (5,708)
Physical contracts 0 0
Interest-rate contracts 0  
Total derivative liabilities (15,178) (5,708)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Derivative assets    
Financial contracts 0 0
Interest-rate contracts 29,596 49,960
Total derivative assets 29,596 49,960
Derivative liabilities    
Financial contracts 0 0
Physical contracts 0 0
Interest-rate contracts (5,290)  
Total derivative liabilities (5,290) 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
Derivative assets    
Financial contracts 23,498 20,203
Interest-rate contracts 0 0
Total derivative assets 23,498 20,203
Derivative liabilities    
Financial contracts (45,145) (48,260)
Physical contracts (1,854) (4,781)
Interest-rate contracts 0  
Total derivative liabilities $ (46,999) $ (53,041)