Lease Commitments - Operating Lease Balance Sheet Information (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Operating leases:
Operating lease assets, net of amortization	$ 85,956	$ 105,133
Current portion of operating lease liabilities	28,230	27,460
Long-term portion of operating lease liabilities	55,106	81,065
PNM
Operating leases:
Operating lease assets, net of amortization	79,808	97,461
Current portion of operating lease liabilities	26,166	25,130
Long-term portion of operating lease liabilities	51,233	75,941
TNMP
Operating leases:
Operating lease assets, net of amortization	5,778	7,206
Current portion of operating lease liabilities	1,988	2,193
Long-term portion of operating lease liabilities	$ 3,579	$ 4,779