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ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Amortization of net loss $ 10,950 $ 10,648 $ 21,900 $ 21,296
Amortization of unrecognized prior service cost (1,142) (1,149) (2,284) (2,298)
Reclassification adjustment, before tax and regulatory adjustments 9,808 9,499 19,616 18,998
Regulatory adjustments (9,537) (9,289) (19,074) (18,579)
Reclassification adjustment, before tax 271 210 542 419
Reclassification adjustment, Tax (68) (81) (419) (161)
Reclassification adjustment, net of tax $ 203 $ 129 $ 123 $ 258