REGULATORY MATTERS Changes in Regulatory Assets and Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Revenue Accruals and Deferrals
Net regulatory liability as of December 31, 2014	$ (56,103)
Net refund of 2013 revenue deferrals and accruals, including accrued interest	35,156
Net revenue accrual for the year ended December 31, 2015	19,876
Net accrued interest payable for the year ended December 31, 2015	(1,493)
Net regulatory liability as of December 31, 2015	$ (2,564)