Lease Commitments - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jan. 16, 2016 lease	Jan. 15, 2015 lease
Operating Leased Assets [Line Items]
Operating lease, payments to lessor upon occurrence of certain events		$ 154.5		$ 154.5
Unamortized cost, right-of-way lease payments		58.0		58.0		$ 60.2
Operating lease, right-of-use assets, amortization expense		0.9	$ 0.9	2.8	$ 2.8
Operating leases, capitalized financing costs, investing activities				2.6	3.0
Finance leases, capitalized financing costs, investing activities				1.8	0.4
Lessee, operating lease, unguaranteed residual value		27.3		27.3
PNM
Operating Leased Assets [Line Items]
Operating leases, capitalized financing costs, investing activities				0.8	1.0
Finance leases, capitalized financing costs, investing activities				0.7	0.2
Lessee, operating lease, unguaranteed residual value		11.3		11.3
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	$ 7.1
TNMP
Operating Leased Assets [Line Items]
Operating leases, capitalized financing costs, investing activities				1.8	1.9
Finance leases, capitalized financing costs, investing activities				1.1	$ 0.2
Lessee, operating lease, unguaranteed residual value		16.0		16.0
Equipment
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		2.1		2.1
Equipment | PNM
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		0.8		0.8
Equipment | TNMP
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset		$ 1.3		$ 1.3