Lease Commitments - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2022 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)	Apr. 02, 2021 lease	Jan. 15, 2016 lease	Jan. 15, 2015 lease
Operating Leased Assets [Line Items]
Operating lease, payments to lessor upon occurrence of certain events		$ 142.2		$ 142.2
Unamortized cost, right-of-way lease payments		51.1		51.1		$ 53.4
Operating lease, right-of-use asset, amortization expense		0.9	$ 0.9	2.9	$ 2.8
Lessee, operating lease, unguaranteed residual value		24.4		24.4
PNM
Operating Leased Assets [Line Items]
Number of leases under which lease term was extended | lease							5
Lessee, operating lease, unguaranteed residual value		11.5		11.5
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
Right-of-way lease payments	$ 7.9
TNMP
Operating Leased Assets [Line Items]
Lessee, operating lease, unguaranteed residual value		12.9		12.9
Equipment
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		2.2		2.2
Equipment | PNM
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		0.9		0.9
Equipment | TNMP
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		$ 1.3		$ 1.3