Fair Value of Financial Instruments (Details-3)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Basis adjustment:
Basis adjustment - on peak	92.40%	91.90%
Basis adjustment - off peak	94.20%	93.40%
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	55.00%	55.00%
Purchased power	25.00%	25.00%
Maximum [Member]
Counterparty fuel mix:
Internal generation	75.00%	75.00%
Purchased power	45.00%	45.00%