Regulatory Matters - Cost Recovery Mechanism and Net Metering (Details)																1 Months Ended	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 01, 2018 USD ($)	May 01, 2018 $ / kWh	Feb. 22, 2018 USD ($)	Feb. 15, 2018 USD ($)	Feb. 09, 2018	Feb. 01, 2018 $ / kWh	Jan. 08, 2018 USD ($)	Nov. 20, 2017 USD ($)	Aug. 19, 2017 $ / kWh	Jun. 01, 2017 USD ($)	Feb. 01, 2017 $ / kWh	Jan. 13, 2017 USD ($)	Dec. 20, 2016 $ / kWh	Feb. 01, 2016 $ / kWh	Jan. 15, 2016 USD ($)	Dec. 31, 2014 penetration_feeder storage_system MW	Jun. 30, 2018 USD ($) MW	Jun. 30, 2018 USD ($) $ / kWh MW	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 29, 2018 USD ($)	Nov. 14, 2017 USD ($)	Sep. 01, 2017 USD ($)	Jan. 27, 2017 USD ($)	Jul. 01, 2016 USD ($)	Jun. 01, 2016 USD ($)	Nov. 25, 2015 USD ($)	Mar. 20, 2015 project
Change in regulatory asset
Deferred fuel and purchased power costs — current period																		$ 50,112,000	$ 21,993,000
Amounts refunded/(charged) to customers																		$ (50,851,000)	13,663,000
Ballot Initiative, proposed required energy supply from renewable sources (as a percent)					50.00%
Ballot initiative, estimate of additional resources (in MW) | MW																	5,500	5,500
Ballot initiative, estimate of incremental capital investment																	$ 10,000,000,000	$ 10,000,000,000
Ballot initiative, statutory fine per violation																	5,000
Palo Verde
Change in regulatory asset
Ballot initiative, estimate of book value and cost, early shut down																	1,900,000,000	1,900,000,000
Four Corners
Change in regulatory asset
Ballot initiative, estimate of book value and cost, early shut down																	$ 1,300,000,000	1,300,000,000
APS
Change in regulatory asset
Deferred fuel and purchased power costs — current period																		50,112,000	21,994,000
Amounts refunded/(charged) to customers																		$ (50,851,000)	13,663,000
Gross-up for revenue requirement of rate regulation																				$ (377,000,000)
RES 2014 | APS | Alternative to AZ Sun Program, Phase 1
Settlement Agreement
Request to build additional utility scale solar, capacity (in MW) | MW																8
RES 2014 | APS | Alternative to AZ Sun Program Phase 2
Settlement Agreement
Request to build additional utility scale solar, capacity (in MW) | MW																2
Number of energy storage systems | storage_system																2
Solar storage system, capacity (in MW) | MW																2
Number of high solar penetration feeders | penetration_feeder																2
Lost Fixed Cost Recovery Mechanisms | APS
Change in regulatory asset
Fixed costs recoverable per residential power lost (in dollars per kWh) | $ / kWh																		0.031
Fixed costs recoverable per non-residential power lost (in dollars per kWh) | $ / kWh																		0.023
Fixed cost recoverable per power lost (in dollars per kWh) | $ / kWh																		0.025
Percentage of retail revenues																	1.00%	1.00%
Amount of adjustment representing prorated sales losses approval												$ 63,700,000			$ 46,400,000
Increase (decrease) in amount of adjustment representing prorated sales losses				$ (3,000,000)								$ 17,300,000			$ 7,900,000
Amount of adjustment representing prorated sales losses pending approval				$ 60,700,000
ACC | APS
Change in regulatory asset
Gross-up for revenue requirement of rate regulation							$ 119,100,000
Provisional income tax benefit			$ 119,100,000
ACC | RES | APS
Settlement Agreement
Plan term																		5 years
ACC | RES 2017 | APS
Settlement Agreement
Amount of approved budget																									$ 150,000,000
ACC | RES 2018 | APS
Settlement Agreement
Amount of proposed budget																			90,000,000		$ 89,900,000
ACC | RES 2018 | APS | Solar Communities
Change in regulatory asset
Program term								3 years
ACC | DSMAC 2015 | APS
Settlement Agreement
Amount of approved budget																											$ 68,900,000
Number of resource savings projects | project																												3
ACC | Demand Side Management Adjustor Charge 2017 | APS
Settlement Agreement
Amount of approved budget																								$ 66,600,000
Amount of proposed budget																										$ 62,600,000
ACC | Demand Side Management Adjustor Charge 2018 | APS
Settlement Agreement
Amount of proposed budget																						$ 52,600,000	$ 52,600,000
ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
Beginning balance																		$ 75,637,000	12,465,000	12,465,000
Deferred fuel and purchased power costs — current period																		50,112,000	21,994,000
Amounts refunded/(charged) to customers																		(50,851,000)	13,663,000
Ending balance																	$ 74,898,000	74,898,000	$ 48,122,000	$ 75,637,000
PSA rate (in dollars per kWh) | $ / kWh						0.004555			0.000555		(0.001348)
Forward component of increase in PSA (in dollars per kWh) | $ / kWh						0.002009			0.000876		(0.001027)
Historical component of increase in PSA (in dollars per kWh) | $ / kWh						0.002546			(0.000321)		(0.000321)
Maximum increase (decrease) in PSA rate | $ / kWh						0.004
Fuel and purchased power costs, excess annual limit																	$ 16,400,000	$ 16,400,000
ACC | Net Metering | APS
Change in regulatory asset
Cost of service, resource comparison proxy method, maximum annual percentage decrease													10.00%
Cost of service for interconnected DG system customers, grandfathered period													20 years
Cost of service for new customers, guaranteed export price period													10 years
First-year export energy price (in dollars per kWh) | $ / kWh													0.129
Second-year export energy price (in dollars per kWh) | $ / kWh		0.116
United States Federal Energy Regulatory Commission | Open Access Transmission Tariff | APS
Change in regulatory asset
Increase (decrease) in annual wholesale transmission rates	$ (22,700,000)									$ 35,100,000
Cost Recovery Mechanisms | ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
PSA rate for prior year (in dollars per kWh) | $ / kWh														0.001678
Minimum | ACC | RES 2018 | APS | Solar Communities
Change in regulatory asset
Required annual capital investment								$ 10,000,000
Maximum | ACC | RES 2018 | APS | Solar Communities
Change in regulatory asset
Required annual capital investment								$ 15,000,000