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FAIR VALUE (Details) - Recurring - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Carrying Amount | Level 1    
Fair value:    
Marketable securities $ 859 $ 1,022
Carrying Amount | Level 2    
Fair value:    
Long-term debt 3,375,431 3,371,679
Estimated Fair Value | Level 1    
Fair value:    
Marketable securities 966 1,151
Estimated Fair Value | Level 2    
Fair value:    
Long-term debt $ 3,797,978 $ 3,616,946