Significant Accounting Policies and Responsibility for Financial Statements (Details)		1 Months Ended		3 Months Ended		6 Months Ended
	Jul. 22, 2019 USD ($)	Jul. 31, 2019 $ / shares MW	Jul. 31, 2018 $ / shares	Jun. 30, 2019 USD ($) $ / shares MW	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares MW	Jun. 30, 2018 USD ($) $ / shares	Sep. 05, 2017 MW
Business Acquisition [Line Items]
Payment defaults under agreements				$ 0		$ 0
Dividends declared per common share (dollars per share) | $ / shares			$ 0.265	$ 0.290	$ 0.265	$ 0.580	$ 0.530
Dividends declared on common stock						$ 23,099,000	$ 21,108,000
NMRD
Business Acquisition [Line Items]
Renewable energy capacity in operating (in mw) | MW				33.9		33.9
Texas-New Mexico Power Company
Business Acquisition [Line Items]
Dividends declared on common stock				$ 4,098,000	$ 9,413,000	$ 14,811,000	10,436,000
PNMR Development
Business Acquisition [Line Items]
Solar generation capacity (in megawatts) | MW				30		30		50
PNMR Development | NMRD
Business Acquisition [Line Items]
Ownership percentage				50.00%		50.00%
AEP OnSite Partners | NMRD
Business Acquisition [Line Items]
Ownership percentage				50.00%		50.00%
PNMR Development and AEP OnSite | NMRD
Business Acquisition [Line Items]
Contribution to construction activities						$ 13,300,000	$ 8,000,000.0
Subsequent event
Business Acquisition [Line Items]
Dividends declared per common share (dollars per share) | $ / shares		$ 0.290
Subsequent event | PNMR Development
Business Acquisition [Line Items]
Solar generation capacity (in megawatts) | MW		1.2
Subsequent event | PNMR Development and AEP OnSite | NMRD
Business Acquisition [Line Items]
Contribution to construction activities	$ 11,000,000.0