Significant Accounting Policies and Responsibility for Financial Statements (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) MW	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Payment defaults under agreements	$ 0
Dividends declared on common stock	23,099,000	$ 21,108,000
Revenues	349,645,000	317,878,000
Construction work in progress	225,485,000		$ 194,427,000
Owners' equity	$ 1,684,679,000		1,688,382,000
NMRD
Business Acquisition [Line Items]
Renewable energy capacity in operating (in mw) | MW	33.9
Texas-New Mexico Power Company
Business Acquisition [Line Items]
Dividends declared on common stock	$ 10,713,000	1,024,000
Revenues	80,327,000	81,646,000
Net earnings	4,098,000	9,413,000
Construction work in progress	75,332,000		51,459,000
Owners' equity	$ 667,478,000	642,794,000	$ 674,093,000	$ 634,405,000
PNMR Development | NMRD
Business Acquisition [Line Items]
Ownership percentage	50.00%
AEP OnSite Partners | NMRD
Business Acquisition [Line Items]
Ownership percentage	50.00%
NMRD
Business Acquisition [Line Items]
Revenues	$ 700,000	400,000
Net earnings	(100,000)	100,000
Cash	8,000,000	2,600,000
Construction work in progress	65,200,000	50,800,000
Accounts payable	6,100,000	200,000
Owners' equity	67,100,000	53,200,000
PNMR Development and AEP OnSite | NMRD
Business Acquisition [Line Items]
Contribution to construction activities	$ 7,000,000	$ 5,000,000