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Fair Value of Derivative and Other Financial Instruments - Non-Derivative Financial Instruments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Jan. 01, 2017
Debt Securities, Available-for-sale [Line Items]
Cumulative effect adjustment				$ 0	$ 10,382,000
Transfers between levels	$ 0	$ 0
Available for sale securities realized impairment losses	(9,400,000)	3,300,000	$ (1,200,000)
Debt securities, unrealized loss position	0
PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	205,738,000
Proceeds from sale of debt securities AFS	984,533,000	637,492,000	522,601,000
Available for sale securities realized gain	19,358,000	36,896,000	46,116,000
Available for sale securities realized loss	(16,624,000)	(12,993,000)	$ (25,430,000)
Impairment losses other than temporary included in AOCI	0
Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	328,242,000	323,524,000
Nuclear Decommissioning Trust \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	287,100,000	293,700,000
Mine Reclamation Trust \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	41,100,000	29,800,000
Cash and cash equivalents \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	11,472,000	52,636,000
Domestic value \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	32,997,000	40,032,000
Domestic growth \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	7,258,000	35,456,000
International and other \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	70,777,000	45,867,000
U.S. Government \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	29,503,000	34,317,000
Municipals \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	53,642,000	48,076,000
Corporate and other \| Measured on a recurring basis \| PNMR and PNM
Debt Securities, Available-for-sale [Line Items]
Fair value	$ 114,158,000	67,140,000
Fixed income investments
Debt Securities, Available-for-sale [Line Items]
Investment allocation targets	65.00%
Scenario, adjustment \| Fixed income investments
Debt Securities, Available-for-sale [Line Items]
Investment allocation targets	54.00%
Retained Earnings
Debt Securities, Available-for-sale [Line Items]
Cumulative effect adjustment		$ 11,200,000		$ 11,208,000	$ 10,382,000