Lease Commitments - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Apr. 02, 2021 lease	Dec. 31, 2020 USD ($)	Jan. 15, 2016 lease	Jan. 15, 2015 lease
Operating Leased Assets [Line Items]
Operating lease, payments to lessor upon occurrence of certain events		$ 146.5
Unamortized cost, right-of-way lease payments		55.1			$ 55.8
Operating lease, right-of-use assets, amortization expense		1.0	$ 0.9
Operating leases, capitalized financing costs, investing activities		0.8	0.9
Finance leases, capitalized financing costs, investing activities		1.0	0.5
Lessee, operating lease, unguaranteed residual value		26.1
PNM
Operating Leased Assets [Line Items]
Operating leases, capitalized financing costs, investing activities		0.2	0.3
Finance leases, capitalized financing costs, investing activities		0.4	0.2
Lessee, operating lease, unguaranteed residual value		11.0
PNM | Subsequent event
Operating Leased Assets [Line Items]
Number of leases under which lease term was extended | lease				5
PNM | Palo Verde Nuclear Generating Station, Unit 1 and 4 Leases
Operating Leased Assets [Line Items]
Number of leases, expiring | lease							4
Number of leases under which lease term was extended | lease							4
Annual lease payments during renewal period		16.5
PNM | Palo Verde Nuclear Generating Station, Unit 2 Leases
Operating Leased Assets [Line Items]
Number of leases, expiring | lease						4
Number of leases under which lease term was extended | lease						1
Annual lease payments during renewal period		1.6
PNM | Navajo Nation
Operating Leased Assets [Line Items]
Annual lease payments		6.0
PNM | Navajo Nation | Subsequent event
Operating Leased Assets [Line Items]
Right-of-way lease payments	$ 7.3
TNMP
Operating Leased Assets [Line Items]
Operating leases, capitalized financing costs, investing activities		0.6	0.6
Finance leases, capitalized financing costs, investing activities		0.6	$ 0.3
Lessee, operating lease, unguaranteed residual value		15.1
Equipment
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		2.3
Equipment | PNM
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		0.9
Equipment | TNMP
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		$ 1.4