Regulation (Narrative) (Details) (USD $)	9 Months Ended			0 Months Ended	9 Months Ended	0 Months Ended			0 Months Ended					0 Months Ended						0 Months Ended		9 Months Ended	0 Months Ended			9 Months Ended		0 Months Ended		9 Months Ended		9 Months Ended	12 Months Ended		0 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Aug. 26, 2014 PUCT Docket No. 42779	Jul. 11, 2014 PUCT Docket No. 41852	Sep. 30, 2014 PUCT Docket No. 41852	May 28, 2014 Fixed Fuel Factor PUCT Docket No. 42384	May 01, 2014 PUCT	Sep. 30, 2014 PUCT	Jan. 14, 2014 NMPRC	Oct. 30, 2013 NMPRC	Jan. 14, 2014 NMPRC	Oct. 30, 2013 NMPRC	Oct. 29, 2013 NMPRC	Nov. 15, 2013 FERC	Nov. 15, 2013 FERC	Nov. 14, 2013 FERC	Jan. 31, 2013 FERC	Jul. 11, 2014 Non-Arbitrage Sales PUCT Docket No. 41852	Jul. 11, 2014 Arbitrage Sales PUCT Docket No. 41852	Mar. 10, 2014 MPS To Caliente Transmission Line PUCT Docket No. 41360 kV	Aug. 18, 2014 MPS In & Out Transmission Line PUCT Docket No. 41359 kV	Sep. 30, 2014 Caliente - Coyote Transmission Line kV	Aug. 18, 2014 MPS To Montwood Transmission Line PUCT Docket No. 41809 kV	Oct. 30, 2013 Nuclear Fuel Financing NMPRC	Nov. 15, 2013 Nuclear Fuel Financing FERC	Sep. 30, 2014 Montana Power Station PUCT	Sep. 30, 2014 Montana Power Station NMPRC	Jul. 11, 2014 Montana Power Station Phase II of Construction PUCT MW	Jun. 11, 2014 Montana Power Station Phase II of Construction NMPRC MW	Sep. 30, 2014 Minimum	Jul. 11, 2014 Maximum PUCT Docket No. 41852	Sep. 30, 2014 Four Corners Generating Station	Feb. 28, 2015 Scenario, Forecast PUCT Docket No. 42384	Sep. 30, 2015 Scenario, Forecast PUCT Docket No. 42779	Apr. 21, 2014 Suit Against Department of Energy for Spent Fuel
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program							$ 2,000,000
Fixed Fuel Factor Revision Period, in Months																														4 months
Increase (Decrease) In Fixed Fuel Factor						10,700,000
Percent Increase (Decrease) In Fixed Fuel Factor						6.90%
Period Of Expected Increases In Prices For Natural Gas																																	12 months
Public Utility Requested Surcharge			19,900,000
Public Utility Requested Surcharge Collecting Period																																		12 months
Settlement Agreement, Counterparty's Name																																			United States Department of Energy
Undercollection of fuel revenues	10,781,000	7,248,000						10,700,000
Reconcilable Fuel Expense				545,300,000
Fuel Reconciliation Period				45 months
Fuel Reconciliation Settlement, Increase (Decrease) in Income Before Tax					2,100,000
Rewards For Operations Requested				3,400,000
Disallowed Reconcilable Fuel Expense				1,750,000
Disallowed Reconcilable Fuel Expense, Reserve				500,000
Intervenor Reimbursed Expenses				100,000
Accrued Reclamation Costs, Allowed				70,000
Date participation is ceased																																Jul. 06, 2016
Customers' Percentage of Off System Sales Margins																		100.00%	50.00%
Retained Percentage Of Off System Sales Margins, Period Covered Start Date				Apr. 01, 2014
Retained Percentage Of Off System Sales Margins, Period Covered End Date				Jun. 30, 2015
Retained Percentage of Off System Sales Margins																															10.00%
Number of power generation units authorized to be constructed																										4	4	2	2
Electric Capacity																				115	115	115	115					88	88
Number Of Transmission Lines	3
Line of Credit Facility, Current Borrowing Capacity											300,000,000	400,000,000	300,000,000		400,000,000	300,000,000
Line of Credit Facility, Expiration Period										4 years				4 years
Debt Instrument, Face Amount												300,000,000			300,000,000									50,000,000	50,000,000
Line of Credit Facility, Increase (Decrease), Net									100,000,000
Line of Credit Facility, Maximum Borrowing Capacity											400,000,000
Line of Credit Facility, Expiration Date									Jan. 14, 2019
Line of Credit renewal option, renewal times									2
Line of Credit renewal term, year									1 year
Refund Related to Dispute																	$ 1,900,000