Financing - Short-term Debt and Liquidity (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($)	Dec. 31, 2017 USD ($) derivative	Jul. 26, 2019 USD ($)	Jul. 22, 2019 USD ($)	Jul. 21, 2019 USD ($)	Feb. 22, 2019 USD ($)	Feb. 21, 2019 USD ($)	Jan. 18, 2019 USD ($)	Dec. 31, 2018 USD ($)
Short-term Debt [Line Items]
Financing capacity						$ 25,000,000.0	$ 24,500,000
Short-term debt	$ 342,400,000								$ 235,900,000
Letters of credit outstanding	4,700,000
PNMR 2018 One-Year Term Loan
Short-term Debt [Line Items]
Short-term debt	$ 150,000,000								150,000,000
Term of loan	1 year
Debt issued	$ 150,000,000.0
PNMR 2018 One-Year Term Loan | PNMR
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	3.15%
PNMR Development Revolving Credit Facility
Short-term Debt [Line Items]
Short-term debt	$ 21,900,000								6,000,000
Variable Rate Short-Term Debt
Short-term Debt [Line Items]
Term of derivatives		4 years
Debt issued		$ 50,000,000.0
PNMR Revolving Credit Facility | PNMR
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	3.67%
PNM Revolving Credit Facility
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	3.53%
PNM 2017 New Mexico Credit Facility
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	3.52%
PNMR Development Revolving Credit Facility | PNMR
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	3.41%
TNMP Revolving Credit Facility
Short-term Debt [Line Items]
Weighted-average interest rate for short-term debt	3.16%
PNM
Short-term Debt [Line Items]
Short-term debt	$ 42,200,000								42,400,000
Letters of credit outstanding	2,500,000
Short-term debt – affiliate	0								19,800,000
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	15,000,000								10,000,000
PNM | PNM 2019 Term Loan
Short-term Debt [Line Items]
Debt issued	250,000,000.0							$ 250,000,000.0
TNMP
Short-term Debt [Line Items]
Short-term debt	55,000,000								17,500,000
Letters of credit outstanding	700,000
Short-term debt – affiliate	1,600,000								100,000
TNMP | TNMP Term Loan Agreement
Short-term Debt [Line Items]
Debt issued	35,000,000.0
PNMR Development
Short-term Debt [Line Items]
Short-term debt – affiliate	200,000								500,000
Revolving credit facility
Short-term Debt [Line Items]
Financing capacity	300,000,000.0
Short-term debt	73,300,000								20,000,000
Revolving credit facility | PNM
Short-term Debt [Line Items]
Financing capacity	400,000,000.0
Short-term debt	27,200,000								32,400,000
Revolving credit facility | TNMP
Short-term Debt [Line Items]
Financing capacity	75,000,000.0
Short-term debt	55,000,000								17,500,000
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]
Other current liabilities	(400,000)
Derivative asset	$ 100,000
Derivative, fair value									$ 1,000,000.0
Subsequent event
Short-term Debt [Line Items]
Financing capacity				$ 50,000,000.0	$ 40,000,000.0
Remaining borrowing capacity			$ 692,700,000
Subsequent event | PNMR
Short-term Debt [Line Items]
Remaining borrowing capacity			227,600,000
Consolidated invested cash			900,000
Subsequent event | PNM
Short-term Debt [Line Items]
Remaining borrowing capacity			368,100,000
Consolidated invested cash			0
Subsequent event | PNM | Lines of credit
Short-term Debt [Line Items]
Remaining borrowing capacity			15,000,000.0
Subsequent event | TNMP
Short-term Debt [Line Items]
Remaining borrowing capacity			74,900,000
Consolidated invested cash			13,800,000
Subsequent event | PNMR Development
Short-term Debt [Line Items]
Short-term debt – affiliate			200,000
Remaining borrowing capacity			$ 7,100,000