Regulatory Matters - Cost Recovery Mechanism and Net Metering (Details) Customer in Thousands																					1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	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	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	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																						$ 239,520,000	$ 166,099,000	$ 254,177,000	$ 191,308,000
Change in regulatory asset
Deferred fuel and purchased power costs — current period																								26,473,000	16,702,000
Amounts charged to customers																								4,815,000	(23,307,000)
Rate plan comparison tool, number of customers | Customer					13
Rate plan comparison tool, inconvenience payment					$ 25
APS
Settlement Agreement
Pre-tax income																						245,669,000	173,513,000	266,439,000	208,115,000
Change in regulatory asset
Deferred fuel and purchased power costs — current period																								26,473,000	16,702,000
Amounts charged to customers																								4,815,000	(23,307,000)
Demand side management funds																														$ 36,000,000
Customer credits																						40,000,000		40,000,000
Customer credits, additional funds																						$ 4,000,000		$ 4,000,000
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																																$ 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																								26,473,000	16,702,000
Amounts charged to customers																								4,815,000	(23,307,000)
Ending balance																						$ 101,425,000	$ 30,559,000	101,425,000	$ 30,559,000
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 | 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
Subsequent Event | ACC | RES 2018 | APS
Settlement Agreement
Amount of proposed budget																												$ 84,700,000
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