FAIR VALUE MEASUREMENTS - Part 1 (Details) - USD ($) $ in Thousands	Dec. 31, 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	800	49,700
Cash posted - offsetting derivative net liability positions under master-netting arrangements	0	29,500
Cash posted - remaining in excess of derivative net liability positions included in consolidated Balance Sheets		20,200
Long-term Debt, Fair Value	9,600,000	9,300,000
Long-term Debt	9,400,000	8,500,000
Fair Value, Measurements, Recurring [Member]
Derivative assets
Financial contracts	47,238	0
Physical contracts	1,142
Interest-rate contracts	19,005	49,960
Total derivative assets	67,385	49,960
Derivative assets netting	(32,739)	(24,455)
Derivative Liabilities
Financial contracts	0	(32)
Physical contracts		(4,781)
Interest-rate contracts	(99,260)
Total derivative liabilities	(99,260)	(4,813)
Derivative liabilities netting	32,739	53,936
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Derivative assets
Financial contracts	79,977	24,455
Physical contracts	1,142
Interest-rate contracts	19,005	49,960
Total derivative assets	100,124	74,415
Derivative Liabilities
Financial contracts	(32,739)	(53,968)
Physical contracts		(4,781)
Interest-rate contracts	(99,260)
Total derivative liabilities	(131,999)	(58,749)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets
Financial contracts	10,812	4,252
Physical contracts	0
Interest-rate contracts	0	0
Total derivative assets	10,812	4,252
Derivative Liabilities
Financial contracts	(2,916)	(5,708)
Physical contracts		0
Interest-rate contracts	0
Total derivative liabilities	(2,916)	(5,708)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets
Financial contracts	0	0
Physical contracts	0
Interest-rate contracts	19,005	49,960
Total derivative assets	19,005	49,960
Derivative Liabilities
Financial contracts	0	0
Physical contracts		0
Interest-rate contracts	(99,260)
Total derivative liabilities	(99,260)	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets
Financial contracts	69,165	20,203
Physical contracts	1,142
Interest-rate contracts	0	0
Total derivative assets	70,307	20,203
Derivative Liabilities
Financial contracts	(29,823)	(48,260)
Physical contracts		(4,781)
Interest-rate contracts	0
Total derivative liabilities	$ (29,823)	$ (53,041)