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Financing - Schedule of maturities and interest rates (Details) - Public Service Company of New Mexico - Senior Unsecured Notes	Jul. 28, 2017 USD ($)
SUNs, Issuance in May 2018, Interest rate of 3.15%
Debt Instrument [Line Items]
Aggregate principal amount	$ 55,000,000.0
Stated percentage	3.15%
Senior SUNs, Issuance in May 2018, Interest rate of 3.45% Note Agreement (SUNs), Private Placement Transaction with Institutional Investors, Issuance in May 2018, Interest Rate of 3.45%
Debt Instrument [Line Items]
Aggregate principal amount	$ 104,000,000
Stated percentage	3.45%
SUNs, Issuance in May 2018, Interest rate of 3.69%
Debt Instrument [Line Items]
Aggregate principal amount	$ 88,000,000
Stated percentage	3.68%
SUNs, Issuance in May 2018, Interest rate of 3.93%
Debt Instrument [Line Items]
Aggregate principal amount	$ 38,000,000
Stated percentage	3.93%
SUNs, Issuance in May 2018, Interest rate of 4.22%
Debt Instrument [Line Items]
Aggregate principal amount	$ 45,000,000
Stated percentage	4.22%
SUNs, Issuance in May 2018, Interest rate of 4.50%
Debt Instrument [Line Items]
Aggregate principal amount	$ 20,000,000
Stated percentage	4.50%
SUNs, Issuance in August 2018, Interest rate of 3.78%
Debt Instrument [Line Items]
Aggregate principal amount	$ 15,000,000
Stated percentage	3.78%
SUNs, Issuance in August 2018, Interest rate of 4.60%
Debt Instrument [Line Items]
Aggregate principal amount	$ 85,000,000
Stated percentage	4.60%
Senior Unsecured Note Agreement (SUNs)
Debt Instrument [Line Items]
Aggregate principal amount	$ 450,000,000.0
Scenario, plan | SUNs, Issuance in May 2018
Debt Instrument [Line Items]
Aggregate principal amount	350,000,000.0
Scenario, plan | SUN's, Issuance in August 2018
Debt Instrument [Line Items]
Aggregate principal amount	$ 100,000,000

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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