Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Services billings: | PNMR to PNM
Related Party Transaction [Line Items]
Amount of related party transaction	$ 97,914	$ 94,606	$ 90,827
Services billings: | PNMR to TNMP
Related Party Transaction [Line Items]
Amount of related party transaction	31,095	28,907	28,109
Services billings: | PNM to TNMP
Related Party Transaction [Line Items]
Amount of related party transaction	382	427	554
Services billings: | TNMP to PNMR
Related Party Transaction [Line Items]
Amount of related party transaction	141	66	41
Services billings: | TNMP to PNM
Related Party Transaction [Line Items]
Amount of related party transaction	154	172	0
Interest billings: | PNMR to PNM
Related Party Transaction [Line Items]
Amount of related party transaction	21	11	54
Interest billings: | PNMR to TNMP
Related Party Transaction [Line Items]
Amount of related party transaction	133	132	276
Interest billings: | PNM to PNMR
Related Party Transaction [Line Items]
Amount of related party transaction	220	150	110
Income tax sharing payments: | PNMR to PNM
Related Party Transaction [Line Items]
Amount of related party transaction	23,391	0	1,450
Income tax sharing payments: | PNMR to TNMP
Related Party Transaction [Line Items]
Amount of related party transaction	0	0	0
Income tax sharing payments: | TNMP to PNMR
Related Party Transaction [Line Items]
Amount of related party transaction	$ 20,686	$ 0	$ 0