Lease Commitments - Narrative (Details) $ in Millions		1 Months Ended	3 Months Ended
	Jan. 15, 2016 USD ($) lease MW	Apr. 30, 2018 USD ($)	Mar. 31, 2019 USD ($) lease	Dec. 31, 2018 USD ($)	Jan. 31, 2016 lease	Jan. 16, 2016 lease	Jan. 15, 2015 lease
Operating Leased Assets [Line Items]
Capitalized operating lease costs			$ 1.1
Capitalized financing lease costs (less than)			0.1
Lessee, operating lease, unguaranteed residual value			22.1
Operating lease, unguaranteed residual value				$ 18.5
PNM
Operating Leased Assets [Line Items]
Capitalized operating lease costs			0.4
Capitalized financing lease costs (less than)			0.1
Lessee, operating lease, unguaranteed residual value			9.9
Operating lease, unguaranteed residual value				$ 7.5
PNM | Navajo Nation
Operating Leased Assets [Line Items]
Annual lease payments			6.0
Right-of-way lease payments		$ 6.9
PNM | Palo Verde Nuclear Generating Station, Unit 1 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
Number of leases under which assets were purchased | lease	3		3		3	3
Leased capacity to be purchased (in megawatts) | MW	32.76
PNM | Palo Verde Nuclear Generating Station, Unit 2 Leases, 31.25 MW
Operating Leased Assets [Line Items]
Number of leases under which assets were purchased | lease	1
Payment to lessors	$ 78.1
PNM | Palo Verde Nuclear Generating Station, Unit 2 Leases, January 15, 2016
Operating Leased Assets [Line Items]
Leased capacity to be purchased (in megawatts) | MW	31.25
PNM | Palo Verde Nuclear Generating Station, Unit 2 Leases, 32.76 MW
Operating Leased Assets [Line Items]
Number of leases under which assets were purchased | lease	2
Payment to lessors	$ 85.2
PNM | Palo Verde Nuclear Generating Station
Operating Leased Assets [Line Items]
Restriction on conveying, transferring, leasing and dividends				5.00%
Loss contingency, lease arrangements (up to)			$ 161.2
TNMP
Operating Leased Assets [Line Items]
Capitalized operating lease costs			0.7
Capitalized financing lease costs (less than)			0.1
Lessee, operating lease, unguaranteed residual value			12.2
Operating lease, unguaranteed residual value				$ 11.0
Equipment
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset			1.6
Equipment | PNM
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset			0.5
Equipment | TNMP
Operating Leased Assets [Line Items]
Operating lease, residual value of leased asset			$ 1.1