Regulatory Matters - Settlement Agreement, Cost Recovery Mechanism and Net Metering (Details)															1 Months Ended		2 Months Ended	9 Months Ended
	Aug. 19, 2017 $ / kWh	Jun. 01, 2017 USD ($)	Mar. 27, 2017 USD ($) $ / kWh	Mar. 26, 2017 $ / kWh	Feb. 01, 2017 $ / kWh	Jan. 13, 2017 USD ($)	Dec. 20, 2016 $ / kWh	Jun. 01, 2016 USD ($)	Feb. 01, 2016 $ / kWh	Jan. 15, 2016 USD ($)	Jan. 01, 2016 USD ($)	Nov. 04, 2014	Jan. 06, 2012 USD ($) $ / kWh	Jun. 01, 2011 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2014 penetration_feeder storage_system MW	Apr. 30, 2014 workshop	Sep. 30, 2017 USD ($) $ / kWh	Sep. 30, 2016 USD ($)	Sep. 01, 2017 USD ($)	Jan. 27, 2017 USD ($)	Dec. 05, 2016 USD ($)	Aug. 05, 2016 USD ($)	Jul. 01, 2016 USD ($)	Apr. 01, 2016 USD ($)	Jan. 12, 2016 USD ($)	Nov. 25, 2015 USD ($)	Jun. 01, 2015 USD ($)	Mar. 20, 2015 project
Change in regulatory asset
Deferred fuel and purchased power costs — current period																		$ 43,348,000	$ 46,185,000
Amounts refunded/(charged) to customers																		18,153,000	(28,366,000)
APS
Change in regulatory asset
Deferred fuel and purchased power costs — current period																		43,348,000	46,185,000
Amounts refunded/(charged) to customers																		$ 18,153,000	(28,366,000)
RES 2014 | APS | Alternative to AZ Sun Program, Phase 1
Settlement Agreement
Request to build additional utility scale solar, capacity (in MW) | MW																8
RES 2014 | APS | Alternative to AZ Sun Program Phase 2
Settlement Agreement
Request to build additional utility scale solar, capacity (in MW) | MW																2
Number of energy storage systems | storage_system																2
Solar storage system, capacity (in MW) | MW																2
Number of high solar penetration feeders | penetration_feeder																2
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
Percentage of retail revenues																		1.00%
Amount of adjustment representing prorated sales losses approval						$ 63,700,000				$ 46,400,000
Increase in amount of adjustment representing prorated sales losses						$ 17,300,000				$ 7,900,000
ACC | Retail Rate Case Filing with Arizona Corporation Commission | APS
Settlement Agreement
Net retail base rate, increase			$ 94,600,000
Non-fuel and non-depreciation base rate, increase			87,200,000
Fuel-related base rate decrease			53,600,000										$ 153,100,000
Base rate increase, changes in depreciation schedules			$ 61,000,000
Approximate percentage of increase in average customer bill			3.28%					5.74%
Approximate percentage of increase in average residential customer bill			4.54%					7.96%
Authorized return on common equity (as a percent)			10.00%
Percentage of debt in capital structure			44.20%
Percentage of common equity in capital structure			55.80%
Rate matter, environmental surcharge cap rate (in dollars per kWh) | $ / kWh			0.00050	0.00016
Rate matter, resource comparison proxy for exported energy (in dollars per kWh) | $ / kWh			0.129
Net retail rate increase								$ 165,900,000						$ 95,500,000
Approximate percentage of increase in average retail customer bill														6.60%
Net change in base rates													0
Non-fuel base rate increase													$ 116,300,000
Current base fuel rate (in dollars per kWh) | $ / kWh													0.03757
Approved base fuel rate (in dollars per kWh) | $ / kWh													0.03207
Estimated amount of transfer of cost recovery for certain renewable energy projects from the RES surcharge to base rates													$ 36,800,000
Change in regulatory asset
Annual revenue requirement impact to trigger pass-through rate adjustment																		$ 5,000,000
Reduced system benefits charge, amount											$ 14,600,000
ACC | Retail Rate Case Filing with Arizona Corporation Commission | APS | AZ Sun Program Phase 2
Settlement Agreement
Public utilities, minimum annual renewable energy standard and tariff			$ 10,000,000
Public utilities, maximum annual renewable energy standard and tariff			$ 15,000,000
ACC | RES | APS
Settlement Agreement
Plan term																		5 years
ACC | RES 2016 | APS
Settlement Agreement
Amount of approved budget																										$ 148,000,000
ACC | RES 2017 | APS
Settlement Agreement
Amount of approved budget																								$ 150,000,000
ACC | RES 2018 | APS
Settlement Agreement
Amount of proposed budget																		$ 90,000,000
ACC | Modernization and Expansion of the Renewal Energy Standard | APS
Settlement Agreement
Proposed renewal energy standard, percent of retail sales															30.00%
Current renewal energy standard, percent of retail sales															15.00%
ACC | DSMAC 2015 | APS
Settlement Agreement
Amount of approved budget																									$ 68,900,000		$ 68,900,000
Number of resource savings projects | project																													3
Additional budget approved																							$ 4,000,000
ACC | DSMC 2016 | APS
Settlement Agreement
Amount of approved budget																												$ 68,900,000
ACC | Electric energy efficiency standard | APS
Settlement Agreement
Number of workshops | workshop																	3
Number of days to convene a workshop												120 days
ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
Beginning balance											$ (9,688,000)							12,465,000	(9,688,000)
Deferred fuel and purchased power costs — current period																		43,348,000	46,185,000
Amounts refunded/(charged) to customers																		18,153,000	(28,365,000)
Ending balance															$ 8,132,000			$ 73,966,000	$ 8,132,000
PSA rate (in dollars per kWh) | $ / kWh	0.000555				(0.001348)
Forward component of increase in PSA (in dollars per kWh) | $ / kWh	0.000876				(0.001027)
Historical component of increase in PSA (in dollars per kWh) | $ / kWh	(0.000321)				(0.000321)
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
ACC | Residential Demand Response, Energy Storage and Load Management Program | APS
Settlement Agreement
Additional budget approved																						$ 4,000,000
ACC | Demand Side Management Adjustor Charge 2017 | APS
Settlement Agreement
Amount of approved budget																					$ 66,600,000
Amount of proposed budget								62,600,000
ACC | Demand Side Management Adjustor Charge 2018 | APS
Settlement Agreement
Amount of proposed budget																				$ 52,600,000
United States Federal Energy Regulatory Commission | Open Access Transmission Tariff | APS
Change in regulatory asset
Increase in annual wholesale transmission rates		$ 35,100,000						$ 24,900,000
Cost Recovery Mechanisms | ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
PSA rate for prior year (in dollars per kWh) | $ / kWh									0.001678