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