Fair Value of Financial Instruments (Details-3)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Basis adjustment:
Basis adjustment - on peak	91.10%	92.30%
Basis adjustment - off peak	93.10%	94.10%
US Treasury Bill Securities [Member]
Deferred compensation plan [Abstract]
Investment interest calculation, investment maturity period (26 weeks)	182 days
Investment interest calculation, monthly compounding rate	1.00%
Investment interest calculation, minimum annual rate compounded monthly	7.00%
Minimum [Member]
Counterparty fuel mix:
Internal generation	50.00%	55.00%
Purchased power	25.00%	25.00%
Maximum [Member]
Counterparty fuel mix:
Internal generation	75.00%	75.00%
Purchased power	50.00%	45.00%