REGULATORY MATTERS Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 16,937	$ 14,736
Non-current regulatory assets	249,264	233,376
Current regulatory liabilities	36,102	44,964
Non-current regulatory liabilities	273,564	$ 254,788
Revenue Deferrals, Including Accrued Interest
Schedule of Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	29,218
Non-current regulatory liabilities	8,645
Revenue Accruals, Including Accrued Interest
Schedule of Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	16,937
Non-current regulatory assets	$ 34,943