Regulation (Narrative) (Details) (USD $)	3 Months Ended	0 Months Ended					0 Months Ended				0 Months Ended							3 Months Ended				0 Months Ended		3 Months Ended
	Mar. 31, 2014	Sep. 07, 2013 PUCT MW	Jan. 14, 2014 NMPRC	Oct. 30, 2013 NMPRC	Sep. 07, 2013 NMPRC MW	Oct. 29, 2013 NMPRC	Nov. 15, 2013 FERC	Nov. 14, 2013 FERC	Jan. 31, 2013 FERC	Sep. 09, 2013 PUCT Docket No. 41803	Sep. 27, 2013 PUCT Docket No. 41852	Sep. 07, 2013 MPS To Caliente Transmission Line PUCT Docket No. 41360 kV	Sep. 07, 2013 MPS In & Out Transmission Line PUCT Docket No. 41359 kV	Sep. 07, 2013 Caliente - Coyote Transmission Line PUCT Docket No. 41359 kV	Sep. 07, 2013 MPS To Montwood Transmission Line PUCT Docket No. 41809 kV	Oct. 30, 2013 Nuclear Fuel Financing NMPRC	Nov. 15, 2013 Nuclear Fuel Financing FERC	Mar. 31, 2014 Montana Power Station PUCT	Mar. 31, 2014 Montana Power Station NMPRC	Mar. 31, 2014 Montana Power Station Phase I of Construction PUCT	Mar. 31, 2014 Montana Power Station Phase I of Construction NMPRC	Sep. 07, 2013 Montana Power Station Phase II of Construction PUCT	Sep. 07, 2013 Montana Power Station Phase II of Construction NMPRC	Mar. 31, 2014 Minimum	Apr. 15, 2014 Subsequent Event PUCT Docket No. 42384
Fixed Fuel Factor Revision Period, in Months																								4 months
Increase (Decrease) In Fixed Fuel Factor										$ 16,900,000															$ 10,700,000
Percent Increase (Decrease) In Fixed Fuel Factor										12.20%															6.90%
Reconcilable Fuel Expense											545,300,000
Fuel Reconciliation Period											45 months
Rewards For Operations Requested											3,400,000
Number of power generation units authorized to be constructed																				2	2	2	2
Number of Power Generation Units																		4	4
Electric Capacity		88			88							115	115	115	115
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