Fair Value of Derivative and Other Financial Instruments - Defined Benefit Plans Disclosure (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Public Service Company of New Mexico | Pension Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 543,601	$ 557,923
Fair value of plan assets at end of year	562,016	543,601
Public Service Company of New Mexico | Pension Plan | Participation in PNMR Master Trust Investments: | Fair Value Measurement [Domain]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	467,965
Fair value of plan assets at end of year	487,498	467,965
Public Service Company of New Mexico | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	543,650
Fair value of plan assets at end of year	562,266	543,650
Public Service Company of New Mexico | Pension Plan | Participation in PNMR Master Trust Investments: | Estimate of Fair Value Measurement
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	75,685
Fair value of plan assets at end of year	74,768	75,685
Public Service Company of New Mexico | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets owned	81,177	73,829
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	72,694	72,952
Fair value of plan assets at end of year	80,356	72,694
Public Service Company of New Mexico | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	2,567
Fair value of plan assets at end of year	437	2,567
Public Service Company of New Mexico | Other Postretirement Benefits | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	9,300
Fair value of plan assets at end of year	10,636	9,300
Public Service Company of New Mexico | Other Postretirement Benefits | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	10,260
Fair value of plan assets at end of year	10,816	10,260
Public Service Company of New Mexico | Other Postretirement Benefits | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	6,338
Fair value of plan assets at end of year	6,710	6,338
Public Service Company of New Mexico | Other Postretirement Benefits | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	26,405
Fair value of plan assets at end of year	31,660	26,405
Public Service Company of New Mexico | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	18,959
Fair value of plan assets at end of year	20,918	18,959
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	129,624
Fair value of plan assets at end of year	140,218	129,624
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets owned	38,881	38,124
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	2,567
Fair value of plan assets at end of year	437	2,567
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	10,260
Fair value of plan assets at end of year	10,816	10,260
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	6,338
Fair value of plan assets at end of year	6,710	6,338
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	18,959
Fair value of plan assets at end of year	20,918	18,959
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	337,989
Fair value of plan assets at end of year	347,089	337,989
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets owned	42,296	35,705
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	9,300
Fair value of plan assets at end of year	10,636	9,300
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	26,405
Fair value of plan assets at end of year	31,660	26,405
Public Service Company of New Mexico | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Corporate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year		719
Actual return on assets sold during the period	1	1
Actual return on assets still held at period end	(7)	19
Purchases	92	0
Sales	(247)	(387)
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	352
Fair value of plan assets at end of year	191	352
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets owned	0	0
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Public Service Company of New Mexico | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Pension Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	60,624	62,082
Fair value of plan assets at end of year	63,499	60,624
Texas-New Mexico Power Company | Pension Plan | Participation in PNMR Master Trust Investments: | Fair Value Measurement [Domain]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	50,901
Fair value of plan assets at end of year	53,273	50,901
Texas-New Mexico Power Company | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	60,630
Fair value of plan assets at end of year	63,533	60,630
Texas-New Mexico Power Company | Pension Plan | Participation in PNMR Master Trust Investments: | Estimate of Fair Value Measurement
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	9,729
Fair value of plan assets at end of year	10,260	9,729
Texas-New Mexico Power Company | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets owned	10,145	8,778
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	8,544	9,111
Fair value of plan assets at end of year	10,002	8,544
Texas-New Mexico Power Company | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	308
Fair value of plan assets at end of year	149	308
Texas-New Mexico Power Company | Other Postretirement Benefits | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	1,279
Fair value of plan assets at end of year	1,597	1,279
Texas-New Mexico Power Company | Other Postretirement Benefits | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	449
Fair value of plan assets at end of year	293	449
Texas-New Mexico Power Company | Other Postretirement Benefits | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	1,089
Fair value of plan assets at end of year	1,410	1,089
Texas-New Mexico Power Company | Other Postretirement Benefits | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	3,060
Fair value of plan assets at end of year	4,011	3,060
Texas-New Mexico Power Company | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	2,593
Fair value of plan assets at end of year	2,685	2,593
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	14,447
Fair value of plan assets at end of year	15,244	14,447
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets owned	4,537	4,439
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	308
Fair value of plan assets at end of year	149	308
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	449
Fair value of plan assets at end of year	293	449
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	1,089
Fair value of plan assets at end of year	1,410	1,089
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	2,593
Fair value of plan assets at end of year	2,685	2,593
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	36,416
Fair value of plan assets at end of year	38,008	36,416
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets owned	5,608	4,339
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	1,279
Fair value of plan assets at end of year	1,597	1,279
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	3,060
Fair value of plan assets at end of year	4,011	3,060
Texas-New Mexico Power Company | Significant Other Observable Inputs (Level 2) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Corporate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year		78
Actual return on assets sold during the period	0	0
Actual return on assets still held at period end	(1)	2
Purchases	10	0
Sales	(26)	(42)
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Pension Plan | Participation in PNMR Master Trust Investments:
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	38
Fair value of plan assets at end of year	21	38
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets owned	0	0
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Texas-New Mexico Power Company | Significant Unobservable Inputs (Level 3) | Other Postretirement Benefits | Mutual funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Fair Value Measurement [Domain]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	518,866
Fair value of plan assets at end of year	540,771	518,866
PNMR
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	604,280
Fair value of plan assets at end of year	625,799	604,280
PNMR | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	20,503
Fair value of plan assets at end of year	7,697	20,503
PNMR | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	38,401
Fair value of plan assets at end of year	42,048	38,401
PNMR | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	36,036
Fair value of plan assets at end of year	37,026	36,036
PNMR | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	18,484
Fair value of plan assets at end of year	19,136	18,484
PNMR | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	27,532
Fair value of plan assets at end of year	25,099	27,532
PNMR | Corporate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	205,419
Fair value of plan assets at end of year	215,535	205,419
PNMR | U.S. Government
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	94,359
Fair value of plan assets at end of year	117,572	94,359
PNMR | Municipals
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	13,970
Fair value of plan assets at end of year	11,438	13,970
PNMR | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	64,162
Fair value of plan assets at end of year	65,220	64,162
PNMR | Private equity funds | Estimate of Fair Value Measurement
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	27,060
Fair value of plan assets at end of year	22,281	27,060
PNMR | Hedge funds | Estimate of Fair Value Measurement
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	42,070
Fair value of plan assets at end of year	45,615	42,070
PNMR | Real estate funds | Estimate of Fair Value Measurement
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	16,284
Fair value of plan assets at end of year	17,132	16,284
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	144,071
Fair value of plan assets at end of year	155,462	144,071
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	20,503
Fair value of plan assets at end of year	7,697	20,503
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	36,036
Fair value of plan assets at end of year	37,026	36,036
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	18,484
Fair value of plan assets at end of year	19,136	18,484
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	69,048
Fair value of plan assets at end of year	91,603	69,048
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipals
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	374,405
Fair value of plan assets at end of year	385,097	374,405
PNMR | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Other Observable Inputs (Level 2) | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	38,401
Fair value of plan assets at end of year	42,048	38,401
PNMR | Significant Other Observable Inputs (Level 2) | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Other Observable Inputs (Level 2) | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Other Observable Inputs (Level 2) | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	27,532
Fair value of plan assets at end of year	25,099	27,532
PNMR | Significant Other Observable Inputs (Level 2) | Corporate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	205,029
Fair value of plan assets at end of year	215,323	205,029
PNMR | Significant Other Observable Inputs (Level 2) | U.S. Government
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	25,311
Fair value of plan assets at end of year	25,969	25,311
PNMR | Significant Other Observable Inputs (Level 2) | Municipals
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	13,970
Fair value of plan assets at end of year	11,438	13,970
PNMR | Significant Other Observable Inputs (Level 2) | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	64,162
Fair value of plan assets at end of year	65,220	64,162
PNMR | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	390
Fair value of plan assets at end of year	212	390
PNMR | Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Unobservable Inputs (Level 3) | International funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Unobservable Inputs (Level 3) | Domestic value
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Unobservable Inputs (Level 3) | Domestic growth
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Unobservable Inputs (Level 3) | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Unobservable Inputs (Level 3) | Corporate
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	390	797
Actual return on assets sold during the period	1	1
Actual return on assets still held at period end	(8)	21
Purchases	102	0
Sales	(273)	(429)
Fair value of plan assets at end of year	212	390
PNMR | Significant Unobservable Inputs (Level 3) | U.S. Government
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Unobservable Inputs (Level 3) | Municipals
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Unobservable Inputs (Level 3) | Other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
PNMR | Significant Unobservable Inputs (Level 3) | Private equity funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	352
Fair value of plan assets at end of year	191	352
PNMR | Significant Unobservable Inputs (Level 3) | Hedge funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	38
Fair value of plan assets at end of year	$ 21	$ 38