Regulatory Matters - Cost Recovery Mechanism and Net Metering (Details)																							1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 01, 2021 $ / kWh	Aug. 20, 2020 USD ($) Customer	Jun. 01, 2020 USD ($)	May 01, 2020 $ / kWh	Feb. 14, 2020 USD ($)	Feb. 01, 2020 USD ($) $ / kWh	Nov. 14, 2019 USD ($) Customer	Oct. 31, 2019 USD ($)	Oct. 29, 2019 USD ($)	Jun. 01, 2019 USD ($)	May 01, 2019 $ / kWh	Apr. 10, 2019	Feb. 15, 2019 USD ($)	Feb. 01, 2019 $ / kWh	Aug. 13, 2018 USD ($)	Jun. 01, 2018 USD ($)	May 01, 2018 $ / kWh	Feb. 15, 2018 USD ($)	Feb. 01, 2018 $ / kWh	Jan. 08, 2018 USD ($)	Nov. 20, 2017 USD ($)	Dec. 20, 2016 $ / kWh	Aug. 31, 2016	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2017 $ / kWh	Jul. 01, 2020 USD ($)	May 15, 2020 USD ($)	May 05, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 01, 2019 USD ($)	Mar. 15, 2019 agreement	Dec. 31, 2018 USD ($)	Jun. 29, 2018 USD ($)	Nov. 14, 2017 USD ($)	Sep. 01, 2017 USD ($)
Settlement Agreement
Pre-tax income																								$ 428,479,000	$ 370,415,000	$ 682,656,000	$ 561,723,000
Change in regulatory asset
Deferred fuel and purchased power costs — current period																										82,679,000	60,911,000
Amounts refunded/(charged) to customers																										9,295,000	(38,601,000)
Rate plan comparison tool, number of customers | Customer		3,800					13,000
Rate plan comparison tool, inconvenience payment		$ 25					$ 25
APS
Settlement Agreement
Pre-tax income																								437,097,000	379,897,000	703,536,000	588,012,000
Change in regulatory asset
Deferred fuel and purchased power costs — current period																										82,679,000	60,911,000
Amounts refunded/(charged) to customers																										9,295,000	(38,601,000)
Demand side management funds																																$ 36,000,000
Customer credits																								43,000,000		43,000,000
Customer credits, additional funds																								$ 7,000,000		$ 7,000,000
Lost Fixed Cost Recovery Mechanisms | APS
Change in regulatory asset
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 pending approval					$ 26,600,000								$ 36,200,000					$ 60,700,000
Increase (decrease) in amount of adjustment representing prorated sales losses					$ (9,600,000)								$ (24,500,000)
ACC | APS
Settlement Agreement
Program term																							2 years
Change in regulatory asset
Gross-up for revenue requirement of rate regulation								$ (184,000,000)							$ 86,500,000					$ 119,100,000
Deferred taxes amortization, period												28 years 6 months
Public Utilities, one-time bill credit									$ 64,000,000
Public Utilities, one-time bill credit, additional benefit									$ 39,500,000
ACC | RES | APS
Settlement Agreement
Plan term																										5 years
ACC | RES 2018 | APS
Settlement Agreement
Amount of proposed budget																														$ 84,700,000				$ 86,300,000			$ 89,900,000
ACC | RES 2018 | APS | Solar Communities
Settlement Agreement
Program term																					3 years
ACC | Demand Side Management Adjustor Charge 2018 | APS
Settlement Agreement
Amount of proposed budget																																						$ 52,600,000	$ 52,600,000
ACC | Demand Side Management Adjustor Charge 2019 | APS
Settlement Agreement
Amount of proposed budget																																				$ 34,100,000
ACC | Demand Side Management Adjustor Charge 2020 | APS
Settlement Agreement
Amount of proposed budget																															$ 51,900,000		$ 51,900,000
ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
Beginning balance																										$ 70,137,000	37,164,000	$ 70,137,000
Deferred fuel and purchased power costs — current period																										82,679,000	60,911,000
Amounts refunded/(charged) to customers																										9,295,000	(38,601,000)
Ending balance																								$ 162,111,000	$ 59,474,000	162,111,000	$ 59,474,000
PSA rate (in dollars per kWh) | $ / kWh						(0.000456)								0.001658					0.004555
PSA rate for prior year (in dollars per kWh) | $ / kWh						(0.002086)								0.000536					0.002009
Forward component of increase in PSA (in dollars per kWh) | $ / kWh						0.001630								0.001122					0.002546
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				0.094							0.105						0.116
United States Federal Energy Regulatory Commission | Environmental Improvement Surcharge | APS
Change in regulatory asset
Increase (decrease) in annual wholesale transmission rates						$ 8,750,000
United States Federal Energy Regulatory Commission | Open Access Transmission Tariff | APS
Change in regulatory asset
Increase (decrease) in annual wholesale transmission rates			$ (6,100,000)							$ 25,800,000						$ (22,700,000)
Retail customer rates			$ 10,900,000							$ 4,700,000						$ (26,900,000)
Cost Recovery Mechanisms | ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
Historical component of increase in PSA (in dollars per kWh) | $ / kWh						(0.002115)								(0.002897)
Cost recovery, number of agreements | agreement																																			2
Forecast | ACC | Net Metering | APS
Change in regulatory asset
Second-year export energy price (in dollars per kWh) | $ / kWh	10.5
Forecast | United States Federal Energy Regulatory Commission | Environmental Improvement Surcharge | APS
Change in regulatory asset
Rate matters, increase (decrease) in cost recovery, excess of annual amount																												(2,000,000.0)
Minimum | ACC | APS
Change in regulatory asset
Operating Results								$ (69,000,000)
Minimum | ACC | RES 2018 | APS | Solar Communities
Settlement Agreement
Required annual capital investment																					$ 10,000,000
Maximum | ACC | RES 2018 | APS | Solar Communities
Settlement Agreement
Required annual capital investment																					$ 15,000,000
Damage from Fire, Explosion or Other Hazard | Minimum | Forecast | APS
Settlement Agreement
Pre-tax income																												20,000,000
Damage from Fire, Explosion or Other Hazard | Maximum | Forecast | APS
Settlement Agreement
Pre-tax income																												$ 30,000,000