FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Cash Flow Hedge - Realized Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Capital Lease Interest Expense	$ 3,356	$ 3,994	$ 10,249
Long-Term Debt Interest Expense	62,015	61,159	60,577
Purchased Power	85,354	124,764	152,922
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Capital Lease Interest Expense	1,000	2,000	2,000
Long-Term Debt Interest Expense	0	0	1,000
Purchased Power	$ 0	$ 1,000	$ 1,000