INCOME TAXES (Reconciliation of Differences between Income Tax Expense and Amount Obtained by Multiplying Pre-Tax Income by U.S. Statutory Federal Income Tax Rate) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory tax rate	35.00%
Federal Income Tax Expense at Statutory Rate	$ 97,000	$ 64,000	$ 70,000
State Income Tax Expense, Net of Federal Deduction	9,000	6,000	8,000
Federal/State Tax Credits	(9,000)	(8,000)	(8,000)
Allowance for Equity Funds Used During Construction	(2,000)	(1,000)	(1,000)
Deferred Tax Asset Valuation Allowance	0	(2,000)	1,000
Impact of Enactment, TCJA	7,000	0	0
Other	(1,000)	0	2,000
Total Federal and State Income Tax Expense	$ 100,763	$ 59,376	$ 71,719