Regulatory Matters - Cost Recovery Mechanisms (Details)																								1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 01, 2021 $ / kWh	Feb. 22, 2021 USD ($)	Feb. 15, 2021 USD ($)	Feb. 01, 2021 USD ($) $ / kWh	Aug. 20, 2020 USD ($) Customer	Jun. 01, 2020 USD ($)	May 01, 2020 $ / kWh	Feb. 14, 2020 USD ($)	Feb. 01, 2020 $ / 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 ($)	Feb. 15, 2018 USD ($)	Feb. 01, 2018 $ / kWh	Jan. 08, 2018 USD ($)	Nov. 20, 2017 USD ($)	Sep. 01, 2017 USD ($) $ / kWh	Mar. 31, 2021 USD ($) $ / kWh	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) program MW	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 ($)	Nov. 14, 2017 USD ($)
Change in regulatory asset
Deferred fuel and purchased power costs — current period																									$ 52,210,000	$ 5,785,000
Amounts refunded to customers																									564,000	1,808,000
Rate plan comparison tool, number of customers | Customer					3,800					13,000
Rate plan comparison tool, inconvenience payment					$ 25					$ 25
APS
Change in regulatory asset
Deferred fuel and purchased power costs — current period																									52,210,000	5,785,000
Amounts refunded to customers																									564,000	1,808,000
Percentage increase under PSA effective for first billing cycle beginning April 2021																								50.00%
Remaining percentage increase under PSA effective for first billing cycle beginning November 2021																								50.00%
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
APS | 2017 Settlement Agreement and its Customer Education and Outreach Plan
Change in regulatory asset
Settlement amount		$ 24,750,000
Settlement amount returned to customers		$ 24,000,000
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			$ 38,500,000					$ 26,600,000								$ 36,200,000			$ 60,700,000
Increase (decrease) in amount of adjustment representing prorated sales losses			$ 11,800,000					$ (9,600,000)								$ (24,500,000)
ACC | APS
Settlement Agreement
Program term																											18 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
Number of programs | program																											2
Solar power capacity (in MW) | MW																											80
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
Revenue requirements																													$ 4,500,000
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																									$ 175,835,000	70,137,000	$ 70,137,000
Deferred fuel and purchased power costs — current period																									52,210,000	5,785,000
Amounts refunded to customers																									564,000	1,808,000
Ending balance																								$ 228,609,000	$ 228,609,000	$ 77,730,000	175,835,000
PSA rate (in dollars per kWh) | $ / kWh				0.003544					(0.000456)								0.001658			0.004555				0.001544
PSA rate for prior year (in dollars per kWh) | $ / kWh				0.003434					(0.002086)								0.000536							(0.004444)
Forward component of increase in PSA (in dollars per kWh) | $ / kWh				0.000110					0.001630								0.001122							0.005988
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
ACC | Demand Side Management Adjustor Charge 2021 | APS
Settlement Agreement
Amount of proposed budget																											$ 63,700,000
United States Federal Energy Regulatory Commission | Environmental Improvement Surcharge | APS
Change in regulatory asset
Increase (decrease) in annual wholesale transmission rates				$ 10,300,000
Rate matters, increase (decrease) in cost recovery, excess of annual amount				$ 1,500,000
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
Retail customer rates						$ 10,900,000							$ 4,700,000
Cost Recovery Mechanisms | ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
Historical component of increase in PSA (in dollars per kWh) | $ / kWh				0.004					(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	0.105
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