Regulation (Narrative) (Details)																		3 Months Ended				12 Months Ended
	May 01, 2018 USD ($)	Apr. 25, 2018 USD ($)	Apr. 13, 2018	Mar. 15, 2018	Mar. 01, 2018 USD ($)	Feb. 15, 2018 USD ($)	Jan. 01, 2018	Dec. 18, 2017 USD ($)	Nov. 01, 2017	Aug. 01, 2017	Jun. 29, 2017	Feb. 22, 2017 USD ($)	Jan. 10, 2017	Sep. 27, 2016 USD ($)	Aug. 25, 2016 USD ($)	Jul. 01, 2016 USD ($)	Jun. 08, 2016 USD ($)	Mar. 31, 2018 USD ($) MW	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 20, 2018 MW
Public Utilities, Requested Rate Decrease, Amount					$ 22,700,000
Nuclear Decommissioning Funding, Annual Recovery																		$ 33,578,000			$ 28,482,000
Over-collection of fuel revenues																		$ 14,190,000				$ 14,190,000	$ 6,225,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent																		21.00%			35.00%		35.00%
PUCT
Over-collection of fuel revenues																		$ 13,300,000				$ 13,300,000
Reconcilable Fuel Expense																		$ 271,300,000
Fuel Reconciliation Period																		24 months
Fuel Over And Under Materiality Threshold Percentage																		4.00%				4.00%
PUCT | Community Solar At Montana Power Station
Electric Capacity | MW																		3
Number of customers subscribed																		1,500				1,500
Property, Plant and Equipment, Operational Date																		May 31, 2017
PUCT | Community Solar at Newman
Electric capacity, requested | MW																								2
PUCT | Solar Facility At Newman Power Station
Electric Capacity | MW																		10
NMPRC
Over-collection of fuel revenues																		$ 900,000				$ 900,000
Reconcilable Fuel Expense																		$ 181,000,000
Fuel Reconciliation Period																		39 months
NMPRC | Holloman
Electric Capacity | MW																		5
FERC
Show Cause Orders issued, Number				2
Number of public utilities affected				48
2016 Energy Efficiency Programs | PUCT
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program						$ 800,000
2016 Energy Efficiency Programs | NMPRC
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program																				$ 400,000
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program																			$ 400,000
Fixed Fuel Factor | PUCT
Increase (Decrease) In Fixed Fuel Factor, Percentage									(19.00%)
Fixed Fuel Factor | PUCT | Subsequent Event
Public Utilities, Requested Rate Increase (Decrease), Percentage			(29.00%)
2017 Energy Efficiency Programs | PUCT | Subsequent Event
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program	$ 1,100,000
2017 Energy Efficiency Programs | NMPRC
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program																$ 400,000
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program												$ 400,000
2015 Energy Efficiency Programs | NMPRC
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program																$ 300,000
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program																					$ 300,000
Public Utilities, Regulatory Rate Making Impact | PUCT
Recovery of rate case expenses															$ 3,100,000
Period To Relate Back New Rates															80 days
Fuel Reconciliation Period							167 days
Public Utilities, Regulatory Rate Making Impact | NMPRC
Public Utilities, Approved Return on Equity, Percentage																	9.48%
Public Utilities, Regulatory Rate Making Impact | Non-Fuel Base Rate | PUCT | Four Corners Generating Station
Public Utilities, Approved Rate Increase (Decrease), Amount															$ 3,700,000
Regulatory Current Asset, End Date for Recovery															Jul. 11, 2017
Public Utilities, Regulatory Rate Making Impact | Non-Fuel Base Rate | NMPRC
Public Utilities, Approved Rate Increase (Decrease), Amount																	$ 600,000
Public Utilities, Regulatory Rate Making Impact | Utility Cost Of Service | NMPRC
Public Utilities, Approved Rate Increase (Decrease), Amount																	$ 500,000
PUCT Docket No. 46831
Recovery of rate case expenses								$ 3,400,000
Surcharge Or Refund Period								3 years
Minimum Distributive Generation Charge								$ 30.00
Nuclear Decommissioning Funding, Annual Recovery								2,100,000
Requested Depreciation Expense, Decrease								$ 1,900,000
Public Utilities, Approved Return on Equity, Percentage								9.65%
PUCT Docket No. 46831 | Four Corners decommissioning [Member]
Decommissioning Costs Increase (Decrease)								$ 5,500,000
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided										7 years
PUCT Docket No. 46831 | Non-Fuel Base Rate
Public Utilities, Approved Rate Increase (Decrease), Amount								$ 14,500,000
PUCT Docket No. 46610 | Fixed Fuel Factor
Increase (Decrease) In Fixed Fuel Factor, Percentage													28.80%
PUCT Docket No. 46308
Reconcilable Fuel Expense														$ 436,600,000
Fuel Reconciliation Period											36 months			36 months
Fuel reconciliation effect, net amount																						$ 5,000,000
Palo Verde Performance Award Period																		3 years
NMPRC Case No. 15-00127-UT
Fuel Reconciliation Period																	24 months
NMPRC Case No. 18-00016-UT | Subsequent Event
Public Utilities, Approved Rate Increase (Decrease), Amount		$ (4,900,000)
Maximum | FERC
Period to comply with order from regulatory agency				60 days