Lease Commitments - Operating Lease Balance Sheet Information (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Operating leases:
Operating lease assets, net of amortization	$ 118,140	$ 131,212
Current portion of operating lease liabilities	26,744	29,068
Long-term portion of operating lease liabilities	91,320	105,512
PNM
Operating leases:
Operating lease assets, net of amortization	109,042	120,585
Current portion of operating lease liabilities	24,039	25,927
Long-term portion of operating lease liabilities	85,028	97,992
TNMP
Operating leases:
Operating lease assets, net of amortization	8,535	9,954
Current portion of operating lease liabilities	2,428	2,753
Long-term portion of operating lease liabilities	$ 5,897	$ 7,039