Significant Accounting Policies and Responsibility for Financial Statements (Details)		3 Months Ended					9 Months Ended
	Aug. 24, 2018 power_purchase_agreement	Sep. 30, 2018 USD ($) $ / shares MW	Jun. 30, 2018 $ / shares	Dec. 31, 2017 USD ($) power_purchase_agreement	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares MW	Sep. 30, 2017 USD ($) $ / shares	Sep. 05, 2017 MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Payment defaults under agreements		$ 0					$ 0
Dividends declared per common share (dollars per share) | $ / shares		$ 0.2650			$ 0.2425		$ 0.7950	$ 0.7275
Dividends declared on common stock							$ 63,325,000
Revenues		$ 422,666,000			$ 419,900,000		1,092,857,000	$ 1,112,398,000
Construction work in progress		227,367,000		$ 245,933,000			227,367,000
Owners' equity		1,771,298,000		1,695,253,000			1,771,298,000
Cash and cash equivalents		$ 34,964,000		3,974,000	$ 43,149,000		$ 34,964,000	43,149,000		$ 5,522,000
NMRD
Business Acquisition [Line Items]
Renewable energy capacity in operating (in mw) | MW		34.3					34.3
Texas-New Mexico Power Company
Business Acquisition [Line Items]
Dividends declared per common share (dollars per share) | $ / shares		$ 0.2650	$ 0.2650		$ 0.2425	$ 0.2425
Dividends declared on common stock							$ 25,804,000	29,700,000
Revenues		$ 91,292,000			$ 92,646,000		260,741,000	257,489,000
Net earnings		16,100,000			$ 14,727,000		40,879,000	$ 34,535,000
Construction work in progress		77,456,000		34,350,000			77,456,000
Owners' equity		649,480,000		634,405,000			649,480,000
Cash and cash equivalents		0		1,700,000			0
NMRD
Business Acquisition [Line Items]
Revenues		1,000,000					2,500,000
Net earnings		500,000					1,000,000
Cash		2,300,000					2,300,000
Construction work in progress		50,400,000					50,400,000
Accounts payable		700,000					700,000
Owners' equity		$ 52,000,000					$ 52,000,000
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%
PNMR Development and AEP OnSite | NMRD
Business Acquisition [Line Items]
Contribution to construction activities							$ 9,000,000
PNM
Business Acquisition [Line Items]
Revenues		$ 331,374,000			$ 327,254,000		832,116,000	$ 854,909,000
Net earnings		77,508,000			60,827,000		112,018,000	104,417,000
Construction work in progress		141,174,000		204,079,000			141,174,000
Owners' equity		1,537,576,000		1,422,174,000			1,537,576,000
Third party deposits						$ 1,000,000				7,200,000	$ 8,200,000
Cash and cash equivalents		$ 31,486,000		1,108,000	$ 39,370,000		$ 31,486,000	$ 39,370,000		1,324,000
Accounting Standards Update 2016-18 | PNM
Business Acquisition [Line Items]
Cash and cash equivalents						$ 1,000,000				$ 1,000,000
Accounting Standards Update 2016-18 | NMCUC
Business Acquisition [Line Items]
Cash and cash equivalents				$ 100,000
Facebook Data Center
Business Acquisition [Line Items]
Number of additional PPAs | power_purchase_agreement	2			3
Power purchase agreement term	25 years			25 years