Income Taxes - Carryforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
New Mexico corporate tax rate being phased in					7.60%
New Mexico Corporate tax rate, effective by 2018				5.90%
Regulatory liability	$ (10,109)	$ (7,132)	$ (1,903)
Income tax expense	1,259	(712)	674
Income Taxes, Impairment of Carryforwards
State tax credit carryforwards	0	0	3,092
State net operating loss carryforwards	819	(311)	5,278
Charitable contribution carryforwards	909	0	2,042
Income Taxes, Reserve Balances
State tax credit carryforwards	2,487	3,986
State net operating loss carryforwards	1,131	361
Charitable contribution carryforwards	952	659
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Federal net operating loss carryforwards	410,400
Federal tax credit carryforwards that expire beginning in 2023	71,800
Public Service Company of New Mexico
Operating Loss Carryforwards [Line Items]
Regulatory liability	(10,109)	(7,132)	(1,903)
Income tax expense	1,179	(804)	470
Income Taxes, Impairment of Carryforwards
State tax credit carryforwards	0	0	0
State net operating loss carryforwards	627	(213)	3,619
Charitable contribution carryforwards	0	0	0
Income Taxes, Reserve Balances
State tax credit carryforwards	0	0
State net operating loss carryforwards	839	248
Charitable contribution carryforwards	0	0
Texas-New Mexico Power Company
Operating Loss Carryforwards [Line Items]
Regulatory liability	0	0	0
Income tax expense	0	0	0
Income Taxes, Impairment of Carryforwards
State tax credit carryforwards	0	0	0
State net operating loss carryforwards	0	0	0
Charitable contribution carryforwards	0	0	$ 0
Income Taxes, Reserve Balances
State tax credit carryforwards	0	0
State net operating loss carryforwards	0	0
Charitable contribution carryforwards	0	$ 0
2015 Electric Rate Case | Public Service Company of New Mexico
Income Taxes, Reserve Balances
Approval to recover impairment of net operating loss carryforward	(2,100)
Recovery period of regulatory asset		2 years
2016 Electric Rate Case | Public Service Company of New Mexico
Income Taxes, Reserve Balances
Approval to recover impairment of net operating loss carryforward	$ (3,300)