Palo Verde and Four Corners (Narrative) (Details) - USD ($) $ in Thousands											9 Months Ended
	Oct. 31, 2016	Jul. 06, 2016	Mar. 31, 2016	Nov. 02, 2015	Jun. 01, 2015	Feb. 17, 2015	Oct. 31, 2014	Oct. 08, 2014	Aug. 18, 2014	Dec. 19, 2012	Sep. 30, 2016	Sep. 30, 2015
Jointly Owned Utility Plant Interests [Line Items]
Proceeds from Sale of Property, Plant, and Equipment											$ 3,251	$ 644
Refund passed through to customers											$ (11,766)	$ 10,934
Four Corners Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Mine Reclamation Liability, Noncurrent		$ 19,500
Public Utilities, Property, Plant and Equipment, Amount of Disposal Adjustments		$ 1,300
Disposal Date		Jul. 06, 2016
Property, Plant and Equipment, Disposals		$ 32,000
Asset Retirement Obligation		7,000
Proceeds from Sale of Property, Plant, and Equipment		4,200
Purchase and Sale Agreement, Execution Date						Feb. 17, 2015
Accounts Payable		$ 1,600
Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Settlement Agreement, Counterparty's Name										U.S. Department of Energy
Lawsuit Filing Date				November 2, 2015			October 31, 2014			December 19, 2012
Settlement Agreement, Date									August 18, 2014
Suit Against Department of Energy for Spent Fuel | Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Settlement, Amount			$ 1,900		$ 6,600			$ 9,100
Refund passed through to customers			$ 1,600		$ 5,800			$ 7,900
Settlement Agreement Award Received, Date			Mar. 31, 2016		Jun. 01, 2015			Oct. 08, 2014
Suit Against Department of Energy for Spent Fuel | Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Settlement, Amount				$ 12,000			$ 42,000		$ 57,400
Minimum | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered				July 1, 2014			July 1, 2011			January 1, 2007
Maximum | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered				June 30, 2015			June 30, 2014			June 30, 2011
Subsequent Event | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Lawsuit Filing Date	October 31, 2016
Subsequent Event | Suit Against Department of Energy for Spent Fuel | Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Claim Amount	$ 1,800
Subsequent Event | Suit Against Department of Energy for Spent Fuel | Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Claim Amount	$ 11,300
Subsequent Event | Minimum | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered	July 1, 2015
Subsequent Event | Maximum | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered	June 30, 2016