SHARE-BASED COMPENSATION Allocation of Share-based Compensation Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs
Amounts capitalized to property, plant and equipment	$ 5,391	$ 4,659	$ 4,731
Total share-based compensation	17,609	14,652	15,666
Total tax benefit recognized in the consolidated statement of operations	5,087	4,182	4,557
Operation and Maintenance Expenses
Employee Service Share-based Compensation, Allocation of Recognized Period Costs
Allocated share-based compensation expense	1,672	1,444	1,617
General and Administrative Expenses
Employee Service Share-based Compensation, Allocation of Recognized Period Costs
Allocated share-based compensation expense	$ 10,546	$ 8,549	$ 9,318