Regulatory Matters - Cost Recovery Mechanism and Net Metering (Details)																	1 Months Ended	9 Months Ended		12 Months Ended
	Aug. 13, 2018 USD ($)	Jun. 01, 2018 USD ($)	May 01, 2018 $ / kWh	Feb. 22, 2018 USD ($)	Feb. 20, 2018	Feb. 15, 2018 USD ($)	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 ($)	Dec. 31, 2014 penetration_feeder storage_system MW	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / kWh	Dec. 31, 2012 $ / kWh	Jun. 29, 2018 USD ($)	Nov. 14, 2017 USD ($)	Sep. 01, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jan. 27, 2017 USD ($)	Jul. 01, 2016 USD ($)	Jun. 01, 2016 USD ($)	Nov. 25, 2015 USD ($) project
Change in regulatory asset
Deferred fuel and purchased power costs — current period																		$ 82,486,000	$ 43,348,000
Amounts refunded/(charged) to customers																		(92,397,000)	18,153,000
Ballot Initiative, proposed required energy supply from renewable sources (as a percent)					50.00%
APS
Change in regulatory asset
Deferred fuel and purchased power costs — current period																		82,486,000	43,348,000
Amounts refunded/(charged) to customers																		$ (92,397,000)	18,153,000
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%
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	$ 86,500,000							$ 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																						$ 89,900,000			$ 90,000,000
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																		82,486,000	43,348,000
Amounts refunded/(charged) to customers																		(92,397,000)	18,153,000
Ending balance																		65,726,000	$ 73,966,000	$ 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
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