Variable Interest Entities (Details)				12 Months Ended
	Feb. 01, 2018 USD ($)	Feb. 01, 2016 USD ($)	Jan. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) lease investor MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Feb. 20, 2018 USD ($)	Jul. 19, 2016 USD ($)	May 31, 2016 MW	Jan. 15, 2016 lease MW	Jan. 15, 2015 lease	Dec. 31, 1986 lease	Dec. 31, 1985 lease
Variable Interest Entity, Statement Of Operation [Abstract]
Earnings attributable to non-controlling interest				$ 15,017,000	$ 14,519,000	$ 14,910,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Current assets				294,420,000	378,039,000
Total assets				6,646,103,000	6,471,080,000	6,009,328,000
Current liabilities				835,644,000	805,108,000
Owners’ equity – non-controlling interest				66,195,000	68,920,000
Public Service Company of New Mexico
Variable Interest Entity, Statement Of Operation [Abstract]
Earnings attributable to non-controlling interest				15,017,000	14,519,000	14,910,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Current assets				243,631,000	314,245,000
Total assets				4,921,563,000	4,867,546,000
Current liabilities				218,496,000	457,361,000
Owners’ equity – non-controlling interest				$ 66,195,000	68,920,000
Number of different institutional investors of trust lessors | investor				5
Accrued lease payments				$ 25,726,000
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Variable Interest Entities, Other financial obligations due to beneficial owner				0
Variable interest entities, Exposure beyond contractual lease payments				0
Variable interest entities, Additional rights to assets beyond use of leased assets				0
Variable interest entity, nonconsolidated, carrying amount, assets				$ 0
Public Service Company of New Mexico | Valencia
Variable Interest Entity [Line Items]
Number of megawatts purchased (in megawatts) | MW				158
Payment for fixed charges				$ 19,600,000	19,300,000	19,200,000
Payment for variable charges				$ 1,300,000	1,100,000	1,600,000
Purchase price, percentage of the book value reduced by related indebtedness				50.00%
Purchase price, percentage of fair market value				50.00%
Variable Interest Entity, Statement Of Operation [Abstract]
Operating revenues				$ 20,887,000	20,371,000	20,687,000
Operating expenses				(5,870,000)	(5,852,000)	(5,777,000)
Earnings attributable to non-controlling interest				15,017,000	14,519,000	$ 14,910,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Current assets				2,688,000	2,551,000
Net property, plant and equipment				64,109,000	66,947,000
Total assets				66,797,000	69,498,000
Current liabilities				602,000	578,000
Owners’ equity – non-controlling interest				$ 66,195,000	68,920,000
Number of years the operating licenses for plants were extended				20 years
Public Service Company of New Mexico | Valencia | Maximum
Variable Interest Entity [Line Items]
Option to purchase a percentage of the plant or VIE (up to )				50.00%
Public Service Company of New Mexico | PVNGS | Palo Verde Nuclear Generating Station
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Accrued lease payments					$ 8,300,000
NM Capital | Coal Supply | San Juan Generating Station
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Loan agreement		$ 125,000,000.0	$ 125,000,000.0
Issuance in letters of credit			30,300,000					$ 30,300,000
NM Capital | Coal Supply | San Juan Coal Company, Westmoreland
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Amount outstanding			$ 125,000,000	$ 56,600,000
Interest receivable				$ 1,000,000
NM Capital | Coal Supply | San Juan Coal Company, Westmoreland | Subsequent Event
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Amount outstanding							$ 51,000,000
Scheduled principal payment	$ 5,600,000
Palo Verde Nuclear Generating Station, Unit 1 Leases | Public Service Company of New Mexico
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Number of leases | lease											4		4
Palo Verde Nuclear Generating Station, Unit 2 Leases | Public Service Company of New Mexico
Variable Interest Entity [Line Items]
Number of megawatts purchased (in megawatts) | MW									64.1	64.1
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract]
Number of leases | lease				3						1		4
Number of leases under which assets were purchased | lease										3