FAIR VALUE MEASUREMENTS - Part 1 (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
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	49,700	67,700
Cash posted - offsetting derivative net liability positions under master-netting arrangements	29,500	51,600
Cash posted - remaining in excess of derivative net liability positions included in consolidated Balance Sheets	20,200	16,100
Long-term Debt, Fair Value	9,300,000	8,800,000
Long-term Debt	8,500,000	8,300,000
Fair Value, Measurements, Recurring [Member]
Derivative assets
Financial contracts	0	951
Interest-rate contracts	49,960	47,457
Total derivative assets	49,960	48,408
Derivative assets netting	(24,455)	(4,760)
Derivative Liabilities
Financial contracts	(32)	0
Physical contracts	(4,781)	(3,022)
Interest-rate contracts		(12,795)
Total derivative liabilities	(4,813)	(15,817)
Derivative liabilities netting	53,936	56,319
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Derivative assets
Financial contracts	24,455	5,711
Interest-rate contracts	49,960	47,457
Total derivative assets	74,415	53,168
Derivative Liabilities
Financial contracts	(53,968)	(56,319)
Physical contracts	(4,781)	(3,022)
Interest-rate contracts		(12,795)
Total derivative liabilities	(58,749)	(72,136)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets
Financial contracts	4,252	1,147
Interest-rate contracts	0	0
Total derivative assets	4,252	1,147
Derivative Liabilities
Financial contracts	(5,708)	(31,458)
Physical contracts	0	0
Interest-rate contracts		0
Total derivative liabilities	(5,708)	(31,458)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets
Financial contracts	0	0
Interest-rate contracts	49,960	47,457
Total derivative assets	49,960	47,457
Derivative Liabilities
Financial contracts	0	0
Physical contracts	0	0
Interest-rate contracts		(12,795)
Total derivative liabilities	0	(12,795)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets
Financial contracts	20,203	4,564
Interest-rate contracts	0	0
Total derivative assets	20,203	4,564
Derivative Liabilities
Financial contracts	(48,260)	(24,861)
Physical contracts	(4,781)	(3,022)
Interest-rate contracts		0
Total derivative liabilities	$ (53,041)	$ (27,883)