Lease Commitments - Operating Lease Balance Sheet Information (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Operating leases:
Operating lease assets, net of amortization	$ 124,717	$ 131,212
Current portion of operating lease liabilities	29,693	29,068
Long-term portion of operating lease liabilities	95,573	105,512
PNM
Operating leases:
Operating lease assets, net of amortization	114,829	120,585
Current portion of operating lease liabilities	26,727	25,927
Long-term portion of operating lease liabilities	88,686	97,992
TNMP
Operating leases:
Operating lease assets, net of amortization	9,271	9,954
Current portion of operating lease liabilities	2,624	2,753
Long-term portion of operating lease liabilities	$ 6,459	$ 7,039