The latest date the public utility can issue debt, in CCYY-MM-DD format.

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The earliest date the public utility can issue debt, in CCYY-MM-DD format.

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Carrying amount as of the balance sheet date of capitalized fuel costs of a regulated entity that are expected to be refunded through rate adjustments within one year or the normal operating cycle, if longer.

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Electric Capacity

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Fixed Fuel Factor revision period, in months

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Fuel Over And Under Materiality Threshold Percentage.

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Fuel reconciliation period

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Increase (decrease) in depreciation expense as a result of a change in the estimated useful life of assets.

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Public utility's entire ownership of its proportionate interest in a jointly owned utility plant, percentage.

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Indicated the number of parties opposing a regulatory matter settlement.

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Number of power generation units authorized to be constructed

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Time period given to public utility (by statutory right) to relate back for consumption, its new rates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Date the public utility's property, plant and equipment became operational, in CCYY-MM-DD format.

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Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program

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Percentage of public utility's interim return on equity allowed while waiting final decision of regulatory agency.

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Amount of public utilities recommended rate increase (decrease) by regulatory agency.

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Public Utilities, Requested Performance Incentive Bonus Related To Energy Efficiency Program

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The execution date of a purchase and sale agreement, in CCYY-MM-DD format.

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Rate case expenses that are amortizable.

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Reconcilable Fuel Expense

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Identifies the customer percentage that each regulatory jurisdiction represents.

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Time period given to public utility (by regulatory agency) to surcharge or to refund customers, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date the debt instrument was issued, in CCYY-MM-DD format.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Date the entity disposed of a business or a group of assets, in CCYY-MM-DD format.

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Carrying value as of the balance sheet date of [accrued] interest payable on all forms of debt, including trade payables, that has been incurred and is unpaid. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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The proportionate share of interest (percent) in a jointly owned utility plant.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of a reclamation and mine closing liability that is associated with a legal obligation for the closure and reclamation of a mine including the removal of buildings, equipment, machinery and other physical remnants of mining, closure of tailings impoundments, leach pads and other mine features, and contouring, covering and revegetation of waste rock piles and other disturbed areas.

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The cash inflow from other long-term borrowings not otherwise defined in the taxonomy (with maturities initially due beyond one year or the normal operating cycle of the entity, if longer).

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Percentage of public utility's approved rate increase (decrease) by regulatory agency.

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Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

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Period end amount of construction work in progress in public utility.

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Amount of public utility's amended requested rate increase (decrease) with regulatory agency.

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Amount of public utility's requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested rate increase (decrease) with regulatory agency.

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Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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Regulation (Narrative) (Details) $ in Thousands																3 Months Ended
Regulation (Narrative) (Details) $ in Thousands	Apr. 29, 2016 USD ($)	Mar. 30, 2016 USD ($)	Mar. 29, 2016 USD ($)	Mar. 24, 2016 USD ($)	Feb. 16, 2016 USD ($)	Nov. 05, 2015 USD ($)	Oct. 28, 2015 USD ($)	Aug. 10, 2015 USD ($)	May. 20, 2015	May. 11, 2015 USD ($)	May. 01, 2015 USD ($)	Dec. 01, 2014 USD ($)	Nov. 14, 2014 USD ($)	Jul. 11, 2014 USD ($)	May. 01, 2014 USD ($)	Mar. 31, 2016 USD ($) MW	Mar. 31, 2015 USD ($)	Jul. 06, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 19, 2015 USD ($)	Oct. 07, 2015 USD ($)
Over-collection of fuel revenues																$ 8,127				$ 4,023
Property, Plant and Equipment, Net																2,313,481				2,286,458
Construction work in progress																296,905				293,796
Regulatory assets																$ 113,542				115,127
Jointly Owned Utility Plant, Proportionate Ownership Share																7.00%
Proceeds from issuance of senior notes																$ 157,052	$ 0
Interest Payable, Current																$ 19,352				$ 12,649
PUCT Docket No. 41852
Reconcilable Fuel Expense														$ 545,300
Fuel Reconciliation Period														45 months
Fixed Fuel Factor \| PUCT Docket No. 44633
Decrease In Fixed Fuel Factor, Percentage									24.00%
Minimum
Fixed Fuel Factor Revision Period, in Months																4 months
PUCT
Period To Relate Back New Rates								155 days
Public Utilities, Annual Depreciation Expense Decrease			$ 8,500
Public Utilities, Interim Return On Equity, Percentage			9.70%
Regulated Jurisdictions, Customers Percentage																76.00%
Number Of Parties Opposing Regulatory Matter Settlement																4
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program											$ 1,000				$ 2,000
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program						$ 1,000							$ 2,000
Fuel Over And Under Materiality Threshold Percentage																4.00%
Over-collection of fuel revenues																$ 5,200
Fuel Reconciliation Period																36 months
PUCT \| Non-Fuel Base Rate
Public Utilities, Requested Rate Increase (Decrease), Amount								$ 71,500
Rate Case Expenses								$ 1,800
Regulatory Asset, Amortization Period								3 years
Public Utilities, Requested Rate Increase, Amended, Amount								$ 63,300
Public Utilities, Interim Rate Increase, Amount		$ 37,000
PUCT \| Four Corners Generating Station
Purchase and Sale Agreement, Execution Date																Feb. 17, 2015
Closing Date of Sale																Jul. 06, 2016
PUCT \| Montana Power Station
Number of power generation units authorized to be constructed																4
PUCT \| Montana Power Station Units 1 and 2
Property, Plant and Equipment, Operational Date																Mar. 20, 2015
PUCT \| Montana Power Station Unit 3
Property, Plant and Equipment, Operational Date																May 03, 2016
PUCT \| Community Solar At Montana Power Station
Electric Capacity \| MW																3
PUCT \| Maximum
Surcharge Or Refund Period								18 months
PUCT \| Four Corners Generating Station \| Non-Fuel Base Rate
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 8,000
PUCT \| Subsequent Event
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program	$ 668
NMPRC
Regulated Jurisdictions, Customers Percentage																24.00%
Over-collection of fuel revenues																$ 2,800
Debt Instrument, Face Amount																						$ 310,000
NMPRC \| Non-Fuel Base Rate
Public Utilities, Requested Rate Increase (Decrease), Amount										$ 8,600
Public Utilities, Requested Rate Increase, Amended, Amount							$ 6,400
Public Utilities, Requested Rate Increase (Decrease), Percentage										7.10%
Public Utilities, Recommended Rate Increase, Amount					$ 640
NMPRC \| Fuel In Base Rate
Public Utilities, Requested Rate Increase (Decrease), Amount										$ (15,400)
Public Utilities, Requested Rate Increase (Decrease), Percentage										(21.50%)
NMPRC \| Four Corners Generating Station
Purchase and Sale Agreement, Execution Date																Feb. 17, 2015
Closing Date of Sale																Jul. 06, 2016
NMPRC \| Montana Power Station
Number of power generation units authorized to be constructed																4
NMPRC \| Montana Power Station Units 1 and 2
Property, Plant and Equipment, Operational Date																Mar. 20, 2015
NMPRC \| Montana Power Station Unit 3
Property, Plant and Equipment, Operational Date																May 03, 2016
NMPRC \| Holloman
Electric Capacity \| MW																5
NMPRC \| Four Corners Generating Station
Jointly Owned Utility Plant, Proportionate Ownership Share																7.00%
FERC
Debt Instrument, Face Amount																					$ 310,000
Line of Credit Facility, Maximum Borrowing Capacity																					400,000
Debt Issuance Authorization Date Range Start																Nov. 15, 2015
Debt Issuance Authorization Date Range End																Nov. 15, 2017
FERC \| Four Corners Generating Station
Jointly Owned Utility Plant Entire Ownership Of Proportionate Interest, Percentage																100.00%
RGRT \| NMPRC
Debt Instrument, Face Amount																						$ 65,000
RGRT \| FERC
Debt Instrument, Face Amount																					$ 65,000
Scenario, Forecast \| PUCT \| Four Corners Generating Station
Property, Plant and Equipment, Net																			$ 29,000
Construction work in progress																			$ 5,000
Asset Retirement Obligation																		$ 7,000
Mine Reclamation and Closing Liability, Noncurrent																		$ 19,300
5.00% Senior Notes, Plus Premium, Due 2044 \| NMPRC
Debt Instrument, Face Amount				$ 150,000
Debt Instrument, Issuance Date				Mar. 24, 2016
Debt Instrument, Interest Rate, Stated Percentage				5.00%
Debt Instrument, Maturity Date				Dec. 01, 2044
Proceeds from issuance of senior notes				$ 158,100
Interest Payable, Current				2,400
Debt Instrument, Unamortized Premium				7,100
5.00% Senior Notes, Plus Premium, Due 2044 \| FERC
Debt Instrument, Face Amount				$ 150,000
Debt Instrument, Issuance Date				Mar. 24, 2016
Debt Instrument, Interest Rate, Stated Percentage				5.00%
Debt Instrument, Maturity Date				Dec. 01, 2044
Proceeds from issuance of senior notes				$ 158,100
Interest Payable, Current				2,400
Debt Instrument, Unamortized Premium				$ 7,100
5.00% Senior Notes, Net of Discount, Due 2044 \| NMPRC
Debt Instrument, Face Amount												$ 150,000
Debt Instrument, Issuance Date												Dec. 01, 2014
Debt Instrument, Interest Rate, Stated Percentage												5.00%
Debt Instrument, Maturity Date												Dec. 01, 2044
5.00% Senior Notes, Net of Discount, Due 2044 \| FERC
Debt Instrument, Face Amount												$ 150,000
Debt Instrument, Issuance Date												Dec. 01, 2014
Debt Instrument, Interest Rate, Stated Percentage												5.00%
Debt Instrument, Maturity Date												Dec. 01, 2044
5.00% Senior Notes, Combined, Due 2044 \| NMPRC
Debt Instrument, Face Amount																$ 300,000
5.00% Senior Notes, Combined, Due 2044 \| FERC
Debt Instrument, Face Amount																300,000
Environmental Restoration Costs \| PUCT
Regulatory assets																$ 7,400