Financing - Short-term Debt and Liquidity (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2017 USD ($) derivative	Apr. 24, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 22, 2019 USD ($)	Jul. 21, 2019 USD ($)
Short-term Debt [Line Items]
Financing capacity					$ 50,000,000.0	$ 40,000,000.0
Short-term debt	$ 300,600,000			$ 185,100,000
Letters of credit outstanding	4,700,000
Senior Unsecured Notes
Short-term Debt [Line Items]
Debt issued	300,000,000.0
Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Term of derivatives		4 years
Debt issued		$ 50,000,000.0
PNMR Development Term Loan
Short-term Debt [Line Items]
Debt issued	90,000,000.0
PNMR 2018 Two Year Term Loan
Short-term Debt [Line Items]
Debt issued	$ 50,000,000.0
Term of loan	2 years
PNMR Revolving Credit Facility | PNMR
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end	2.15%
PNM
Short-term Debt [Line Items]
Short-term debt	$ 60,000,000			58,000,000
Letters of credit outstanding	2,500,000
Long-term debt	100,300,000
PNM | Lines of credit
Short-term Debt [Line Items]
NMPRC approved credit facility	40,000,000.0
Short-term debt	$ 30,000,000			10,000,000
PNM | PNM Revolving Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end	2.35%
PNM | PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end	2.12%
PNM | PNMR Development Revolving Credit Facility
Short-term Debt [Line Items]
Letters of credit outstanding	$ 0
TNMP
Short-term Debt [Line Items]
Short-term debt	70,000,000			15,000,000
Letters of credit outstanding	$ 100,000
TNMP | TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Line of credit facility, interest rate at period end	1.74%
Revolving credit facility
Short-term Debt [Line Items]
Financing capacity	$ 300,000,000.0
Short-term debt	170,600,000			112,100,000
Revolving credit facility | PNM
Short-term Debt [Line Items]
Financing capacity	400,000,000.0
Short-term debt	30,000,000			48,000,000
Revolving credit facility | TNMP
Short-term Debt [Line Items]
Financing capacity	75,000,000.0
Short-term debt	70,000,000			15,000,000
Revolving credit facility | TNMP | First mortgage bonds
Short-term Debt [Line Items]
Collateral amount	75,000,000.0
Interest rate contract
Short-term Debt [Line Items]
Number of derivatives | derivative		3
Interest rate 1 | Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate		1.926%
Interest rate 2 | Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate		1.823%
Interest rate 3 | Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Fixed interest rate		1.629%
Interest rate 3 | Level 2 | Cash Flow Hedge
Short-term Debt [Line Items]
Current derivative liability	$ 2,500,000			$ 400,000
Subsequent event
Short-term Debt [Line Items]
Remaining borrowing capacity			$ 670,000,000.0
Subsequent event | PNMR
Short-term Debt [Line Items]
Remaining borrowing capacity			162,600,000
Consolidated invested cash			900,000
Subsequent event | PNM
Short-term Debt [Line Items]
Remaining borrowing capacity			382,500,000
Consolidated invested cash			1,900,000
Subsequent event | PNM | Lines of credit
Short-term Debt [Line Items]
Remaining borrowing capacity			10,000,000.0
Subsequent event | TNMP
Short-term Debt [Line Items]
Remaining borrowing capacity			74,900,000
Consolidated invested cash			21,900,000
Subsequent event | PNMR Development
Short-term Debt [Line Items]
Remaining borrowing capacity			$ 40,000,000.0