Regulatory Matters - Cost Recovery Mechanism and Net Metering (Details)																					1 Months Ended	9 Months Ended		12 Months Ended
	Oct. 31, 2019 USD ($)	Oct. 29, 2019 USD ($)	Jun. 01, 2019 USD ($)	Apr. 10, 2019 USD ($)	Feb. 15, 2019 USD ($)	Feb. 01, 2019 $ / kWh	Aug. 13, 2018 USD ($)	Jun. 01, 2018 USD ($)	May 01, 2018 $ / kWh	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	Aug. 31, 2016	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2017 USD ($) $ / kWh	Dec. 31, 2012 $ / kWh	Jul. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 29, 2018 USD ($)	Nov. 14, 2017 USD ($)	Sep. 01, 2017 USD ($)	Jun. 30, 2017 USD ($)
Change in regulatory asset
Deferred fuel and purchased power costs — current period																						$ 60,911,000	$ 82,486,000
Amounts charged to customers																						(38,601,000)	(92,397,000)
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																						60,911,000	82,486,000
Amounts charged to customers																						$ (38,601,000)	(92,397,000)
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 approval																		$ 63,700,000
Amount of adjustment representing prorated sales losses pending approval					$ 36,200,000						$ 60,700,000
Increase (decrease) in amount of adjustment representing prorated sales losses					$ (24,500,000)						$ (3,000,000)
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
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			$ 90,000,000
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																												$ 34,100,000		$ 52,600,000	$ 52,600,000
ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
Beginning balance																						$ 37,164,000	75,637,000	$ 37,164,000
Deferred fuel and purchased power costs — current period																						60,911,000	82,486,000
Amounts charged to customers																						(38,601,000)	(92,397,000)
Ending balance																						59,474,000	$ 65,726,000		$ 75,637,000
PSA rate (in dollars per kWh) | $ / kWh						0.001658						0.004555			0.000555		(0.001348)
PSA rate for prior year (in dollars per kWh) | $ / kWh						0.000536						0.002009			0.000876		(0.001027)
Forward component of increase in PSA (in dollars per kWh) | $ / kWh						0.001122						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			$ (4,900,000)					$ (22,700,000)								$ (35,100,000)
Cost Recovery Mechanisms | ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
Historical component of increase in PSA (in dollars per kWh) | $ / kWh						(0.002897)														0.001678
Forecast | ACC | APS
Change in regulatory asset
Operating Results																								$ 10,000,000
Minimum | ACC | RES 2018 | APS | Solar Communities
Settlement Agreement
Required annual capital investment														$ 10,000,000
Minimum | Forecast | ACC | APS
Change in regulatory asset
Operating Results	$ (69,000,000)
Maximum | ACC | RES 2018 | APS | Solar Communities
Settlement Agreement
Required annual capital investment														$ 15,000,000
Subsequent Event | ACC | APS
Change in regulatory asset
Public Utilities, one-time bill credit		$ 64,000,000
Public Utilities, one-time bill credit, additional benefit		$ 39,500,000