REGULATORY MATTERS (2017 Rate Order) (Details) - Arizona Corporation Commission - USD ($) $ in Millions	1 Months Ended
	Feb. 28, 2017	Sep. 30, 2016
Public Utilities, General Disclosures [Line Items]
Approved Return on Original Cost Rate Base, Percentage	7.04%
Approved Cost of Equity, Percentage	9.75%
Approved Cost of Debt Component, Percentage	4.32%
Non-fuel Component of Base Rate
Public Utilities, General Disclosures [Line Items]
Non-fuel base rate increase over adjusted test year revenues	$ 81.5
Springerville Unit One
Public Utilities, General Disclosures [Line Items]
Purchased undivided interest in Springerville Unit 1		50.50%
Springerville Unit One | Non-fuel Component of Base Rate
Public Utilities, General Disclosures [Line Items]
Unit 1 Operating costs included in base rate increase	$ 15.0