Regulatory Matters - Cost Recovery Mechanism and Net Metering (Details) Customer in Thousands																		1 Months Ended	3 Months Ended		12 Months Ended
	Feb. 14, 2020 USD ($)	Feb. 01, 2020 USD ($) $ / kWh	Nov. 14, 2019 USD ($) Customer	Oct. 31, 2019 USD ($)	Oct. 29, 2019 USD ($)	Jun. 01, 2019 USD ($)	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	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2017 $ / kWh	Dec. 31, 2012 $ / kWh	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																			$ 14,657,000	$ 25,209,000
Change in regulatory asset
Deferred fuel and purchased power costs — current period																			5,785,000	(16,709,000)
Amounts charged to customers																			1,808,000	(12,872,000)
Rate plan comparison tool, number of customers | Customer			13
Rate plan comparison tool, inconvenience payment			$ 25
APS
Settlement Agreement
Pre-tax income																			20,770,000	34,602,000
Change in regulatory asset
Deferred fuel and purchased power costs — current period																			5,785,000	(16,709,000)
Amounts charged to customers																			$ 1,808,000	(12,872,000)
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 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
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																			5,785,000	(16,709,000)
Amounts charged to customers																			1,808,000	(12,872,000)
Ending balance																			77,730,000	$ 7,583,000
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
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 | Environmental Improvement Surcharge [Member] | 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						$ 4,900,000					$ (22,700,000)
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 [Member] | 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 | APS
Change in regulatory asset
Demand side management funds																								$ 36,000,000
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