Jointly-Owned Facilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Jul. 06, 2016
4CA
Interests in jointly-owned facilities
Plant in Service	$ 141,000
Accumulated Depreciation	83,000
Construction work in progress	$ 25,000
Palo Verde Units 1 and 3 | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	29.10%
Plant in Service	$ 1,872,104
Accumulated Depreciation	1,092,049
Construction work in progress	$ 24,257
Palo Verde Unit 2 | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	16.80%
Plant in Service	$ 619,263
Accumulated Depreciation	364,516
Construction work in progress	$ 14,672
Palo Verde Common | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	28.00%
Plant in Service	$ 726,223
Accumulated Depreciation	262,065
Construction work in progress	46,577
Palo Verde Sale Leaseback | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Plant in Service	351,050
Accumulated Depreciation	241,405
Construction work in progress	$ 0
Four Corners Generating Station | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	63.00%
Plant in Service	$ 1,196,683
Accumulated Depreciation	568,304
Construction work in progress	$ 240,514
Cholla Common Facilities | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	50.50%
Plant in Service	$ 180,907
Accumulated Depreciation	69,633
Construction work in progress	$ 1,091
ANPP 500kV System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	34.00%
Plant in Service	$ 130,767
Accumulated Depreciation	46,400
Construction work in progress	$ 684
Navajo Southern System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	27.50%
Plant in Service	$ 85,299
Accumulated Depreciation	28,915
Construction work in progress	$ 180
Palo Verde — Yuma 500kV System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	18.10%
Plant in Service	$ 14,765
Accumulated Depreciation	6,614
Construction work in progress	$ 486
Four Corners Switchyards | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	63.20%
Plant in Service	$ 66,386
Accumulated Depreciation	12,605
Construction work in progress	$ 327
Phoenix — Mead System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	17.10%
Plant in Service	$ 39,383
Accumulated Depreciation	17,600
Construction work in progress	$ 41
Palo Verde — Rudd 500kV System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	50.00%
Plant in Service	$ 97,600
Accumulated Depreciation	23,884
Construction work in progress	$ 245
Morgan — Pinnacle Peak System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	64.60%
Plant in Service	$ 117,721
Accumulated Depreciation	14,569
Construction work in progress	$ 1
Round Valley System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	50.00%
Plant in Service	$ 515
Accumulated Depreciation	141
Construction work in progress	$ 0
Palo Verde — Morgan System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	90.90%
Plant in Service	$ 137,887
Accumulated Depreciation	3,948
Construction work in progress	$ 94,350
Hassayampa — North Gila System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	80.00%
Plant in Service	$ 142,541
Accumulated Depreciation	6,953
Construction work in progress	$ 0
Cholla 500kV Switchyard | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	85.70%
Plant in Service	$ 5,243
Accumulated Depreciation	1,312
Construction work in progress	$ 190
Saguaro 500kV Switchyard | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	60.00%
Plant in Service	$ 20,473
Accumulated Depreciation	12,574
Construction work in progress	$ 0
Kyrene - Knox System | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Percent Owned	50.00%
Plant in Service	$ 578
Accumulated Depreciation	297
Construction work in progress	$ 0
Navajo Plant | ARIZONA PUBLIC SERVICE COMPANY
Interests in jointly-owned facilities
Ownership interest by noncontrolling owners (as a percent)	14.00%
El Paso's Interest in Four Corners | 4CA
Interests in jointly-owned facilities
Ownership interest acquired (as a percent)	7.00%	7.00%