INCOME TAXES - Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income taxes statutory rate	35.00%	35.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	3.20%	4.20%	1.40%
Amortization of investment tax credits related to rate-regulated assets		(0.10%)	(0.20%)
Legislative change - federal	2.60%
Other		0.20%	(1.10%)
Effective income tax rate	40.80%	39.30%	35.10%