Lease Commitments - Operating Lease Balance Sheet Information (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Operating leases:
Operating lease assets, net of amortization	$ 111,638	$ 131,212
Current portion of operating lease liabilities	26,826	29,068
Long-term portion of operating lease liabilities	81,933	105,512
PNM
Operating leases:
Operating lease assets, net of amortization	103,251	120,585
Current portion of operating lease liabilities	24,282	25,927
Long-term portion of operating lease liabilities	76,252	97,992
TNMP
Operating leases:
Operating lease assets, net of amortization	7,876	9,954
Current portion of operating lease liabilities	2,341	2,753
Long-term portion of operating lease liabilities	$ 5,311	$ 7,039