Significant Accounting Policies and Responsibility for Financial Statements (Details)	1 Months Ended	3 Months Ended			6 Months Ended
	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) $ / shares MW	Dec. 31, 2017 USD ($) power_purchase_agreement	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares MW	Jun. 30, 2017 USD ($) $ / shares	Sep. 30, 2017	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	$ 0.53	$ 0.4850
Dividends declared on common stock					$ 21,108,000
Revenues		$ 352,313,000		$ 362,320,000	670,191,000	$ 692,498,000
Construction work in progress		220,065,000	$ 245,933,000		220,065,000
Owners' equity		1,722,715,000	1,695,253,000		1,722,715,000
Cash and cash equivalents		$ 4,889,000	3,974,000	2,193,000	$ 4,889,000	2,193,000		$ 5,522,000
NMRD
Business Acquisition [Line Items]
Renewable energy capacity in operating (in mw) | MW		31.8			31.8
PNM
Business Acquisition [Line Items]
Revenues		$ 264,511,000		276,097,000	$ 500,742,000	527,655,000
Net earnings		26,672,000		26,932,000	34,510,000	43,591,000
Construction work in progress		114,535,000	204,079,000		114,535,000
Owners' equity		1,458,012,000	1,422,174,000		1,458,012,000
Third party deposits				1,000,000		1,000,000		7,200,000	$ 8,200,000
Cash and cash equivalents		$ 2,874,000	1,108,000	$ 26,000	2,874,000	26,000		1,324,000
Texas-New Mexico Power Company
Business Acquisition [Line Items]
Dividends declared per common share (dollars per share) | $ / shares		$ 0.2650		$ 0.2425
Dividends declared on common stock					10,436,000	17,500,000
Revenues		$ 87,802,000		$ 86,223,000	169,449,000	164,843,000
Net earnings		15,367,000		12,204,000	24,779,000	19,808,000
Construction work in progress		94,810,000	34,350,000		94,810,000
Owners' equity		648,748,000	634,405,000		648,748,000
Cash and cash equivalents		$ 0	1,700,000		$ 0
PNMR Development
Business Acquisition [Line Items]
Solar generation capacity (in megawatts) | MW		30			30
PNMR Development | NMRD
Business Acquisition [Line Items]
Ownership percentage							50.00%
NMRD
Business Acquisition [Line Items]
Revenues		$ 1,100,000			$ 1,500,000
Net earnings		400,000			500,000
Cash		2,000,000			2,000,000
Construction work in progress		47,900,000			47,900,000
Accounts payable		400,000			400,000
Owners' equity		$ 49,500,000			49,500,000
PNMR Development and AEP OnSite | NMRD
Business Acquisition [Line Items]
Contribution to construction activities					$ 8,000,000
Accounting Standards Update 2016-18 | PNM
Business Acquisition [Line Items]
Cash and cash equivalents				$ 1,000,000		$ 1,000,000		$ 1,000,000
Accounting Standards Update 2016-18 | NMCUC
Business Acquisition [Line Items]
Cash and cash equivalents			$ 100,000
Subsequent event | Texas-New Mexico Power Company
Business Acquisition [Line Items]
Dividends declared on cash	$ 15,400,000
Facebook Data Center
Business Acquisition [Line Items]
Number of additional PPAs | power_purchase_agreement			3
Power purchase agreement term			25 years