Pension and Other Postretirement Benefits - Assumed Health Care Cost Trend Rates and Impact of a One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Details) - Public Service Company of New Mexico - Other Postretirement Benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]
Health care cost trend rate assumed for next year	6.50%	6.80%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate)	5.00%	5.00%
Year that the rate reaches the ultimate trend rate	2024	2024
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]
1-Percentage-Point Increase, Effect on total of service and interest cost	$ 72
1-Percentage-Point Increase, Effect on APBO	1,452
1-Percentage-Point Decrease, Effect on total of service and interest cost	(111)
1-Percentage-Point Decrease, Effect on APBO	$ (2,235)