Regulation (Narrative) (Details) $ in Thousands																3 Months Ended			9 Months Ended		12 Months Ended
	Oct. 28, 2016 USD ($)	Sep. 27, 2016 USD ($)	Aug. 25, 2016 USD ($)	Jul. 06, 2016 USD ($)	Jun. 08, 2016 USD ($)	Apr. 29, 2016 USD ($)	Mar. 24, 2016 USD ($)	Nov. 05, 2015 USD ($)	Oct. 28, 2015 USD ($)	Aug. 10, 2015 USD ($)	May 20, 2015	May 11, 2015 USD ($)	May 01, 2015 USD ($)	Feb. 17, 2015	Dec. 01, 2014 USD ($)	Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) MW	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Oct. 19, 2015 USD ($)	Oct. 07, 2015 USD ($)
Operating revenues																$ 323,225		$ 289,713	$ 698,899	$ 672,967	$ 875,801	$ 869,530
Income before income taxes																117,770		81,340	142,535	116,359	142,989	122,790
Net income																74,636		$ 56,740	91,112	81,270	91,760	$ 85,511
Deferred Fuel Cost																8,962			8,962		8,962		$ 0
Over-collection of fuel revenues																1,219			1,219		1,219		4,023
Jointly Owned Utility Plant, Proportionate Ownership Share				7.00%
Proceeds from issuance of senior notes																			157,052	$ 0
Interest Payable, Current																$ 19,349			$ 19,349		$ 19,349		$ 12,649
Four Corners Generating Station
Purchase and Sale Agreement, Execution Date														Feb. 17, 2015
Closing Date of Sale				Jul. 06, 2016
Mine Reclamation Liability, Noncurrent				$ 19,500
PUCT
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program						$ 668							$ 1,000
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program								$ 1,000
Fuel Over And Under Materiality Threshold Percentage																4.00%			4.00%		4.00%
Deferred Fuel Cost																$ 8,900			$ 8,900		$ 8,900
PUCT | Four Corners Generating Station
Purchase and Sale Agreement, Execution Date																			Feb. 17, 2015
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 | Montana Power Station Unit 4
Property, Plant and Equipment, Operational Date																			Sep. 15, 2016
PUCT | Community Solar At Montana Power Station
Electric Capacity | MW																			3
PUCT | Minimum
Fixed Fuel Factor Revision Period, in Months																			4 months
PUCT | Four Corners Generating Station
Closing Date of Sale				Jul. 06, 2016
Mine Reclamation Liability, Noncurrent																7,500			$ 7,500		7,500
PUCT | Subsequent Event
Public Utilities Approved Performance Incentive Bonus Related To Energy Efficiency Program	$ 500
Public Utilities Requested Performance Incentive Bonus Related To Energy Efficiency Program Reduction, Amount	$ 155
NMPRC
Over-collection of fuel revenues																$ 1,300			$ 1,300		$ 1,300
Debt Instrument, Face Amount																									$ 310,000
NMPRC | Four Corners Generating Station
Purchase and Sale Agreement, Execution Date																			Feb. 17, 2015
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 | Montana Power Station Unit 4
Property, Plant and Equipment, Operational Date																			Sep. 15, 2016
NMPRC | Holloman
Electric Capacity | MW																			5
NMPRC | Four Corners Generating Station
Jointly Owned Utility Plant, Proportionate Ownership Share																7.00%			7.00%		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
Closing Date of Sale				Jul. 06, 2016
Jointly Owned Utility Plant Entire Ownership Of Proportionate Interest, Percentage																100.00%			100.00%		100.00%
RGRT | NMPRC
Debt Instrument, Face Amount																									$ 65,000
RGRT | FERC
Debt Instrument, Face Amount																								$ 65,000
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			$ 300,000		$ 300,000
5.00% Senior Notes, Combined, Due 2044 | FERC
Debt Instrument, Face Amount																300,000			$ 300,000		$ 300,000
Electric Generation Equipment | PUCT | Montana Power Station
Number of power generation units authorized to be constructed																			4
Electric Generation Equipment | NMPRC | Montana Power Station
Number of power generation units authorized to be constructed																			4
Non-Fuel Base Rate | PUCT
Public Utilities, Requested Rate Increase (Decrease), Amount										$ 71,500
Public Utilities, Requested Rate Increase, Amended, Amount										$ 63,300
Non-Fuel Base Rate | NMPRC
Public Utilities, Requested Rate Increase (Decrease), Amount												$ 8,600
Public Utilities, Requested Rate Increase, Amended, Amount									$ 6,400
Public Utilities, Approved Rate Increase, Amount					$ 1,100
Public Utilities, Approved Return on Equity, Percentage					9.48%
Public Utilities, Requested Rate Increase (Decrease), Percentage												7.10%
Public Utilities, Regulatory Rate Making Impact | PUCT
Public Utilities, Depreciation Expense Decrease			$ 8,500													7,400			$ 7,400
Public Utilities, Approved Return on Equity, Percentage			9.70%
Recovery of rate case expenses			$ 3,100
Period To Relate Back New Rates			80 days
Operating revenues																34,800
Increase in other expenses																1,900
Income before income taxes																40,300
Net income																$ 23,300
Public Utilities, Regulatory Rate Making Impact | Non-Fuel Base Rate | PUCT
Public Utilities, Approved Rate Increase, Amount			$ 37,000
Public Utilities, Regulatory Rate Making Impact | Non-Fuel Base Rate | PUCT | Four Corners Generating Station
Public Utilities, Approved Rate Increase, Amount			$ 3,700
Regulatory Current Asset, End Date for Recovery			Jul. 12, 2017
Public Utilities, Regulatory Rate Making Impact | Relate Back Of Rates Revenue | PUCT
Operating revenues																	$ 4,800
PUCT Docket No. 44633 | Fixed Fuel Factor
Decrease In Fixed Fuel Factor, Percentage											24.00%
PUCT Docket No. 46308
Reconcilable Fuel Expense		$ 436,600
Fuel Reconciliation Period		36 months