Regulatory Matters - Cost Recovery Mechanism and Net Metering (Details)																			1 Months Ended		3 Months Ended		12 Months Ended
	Feb. 22, 2018 USD ($)	Feb. 15, 2018 USD ($)	Feb. 09, 2018	Feb. 01, 2018 $ / kWh	Jan. 08, 2018 USD ($)	Jan. 03, 2018	Nov. 20, 2017 USD ($)	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 ($)	Jun. 01, 2011 USD ($)	Apr. 30, 2018 USD ($)	Dec. 31, 2014 penetration_feeder storage_system MW	Mar. 31, 2018 USD ($) $ / kWh	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Nov. 14, 2017 USD ($)	Sep. 01, 2017 USD ($)	Jan. 27, 2017 USD ($)	Jul. 01, 2016 USD ($)	Nov. 25, 2015 USD ($)	Mar. 20, 2015 project
Change in regulatory asset
Deferred fuel and purchased power costs — current period																					$ 18,950,000	$ 988,000
Amounts refunded/(charged) to customers																					(20,002,000)	4,172,000
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																					18,950,000	988,000
Amounts refunded/(charged) to customers																					$ (20,002,000)	4,172,000
Gross-up for revenue requirement of rate regulation																							$ 377,000,000
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
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				$ 46,400,000
Increase in amount of adjustment representing prorated sales losses		$ 3,000,000											$ 17,300,000				$ 7,900,000
Amount of adjustment representing prorated sales losses pending approval		$ 60,700,000
ACC | APS
Change in regulatory asset
Gross-up for revenue requirement of rate regulation					$ (119,100,000)
Provisional income tax expense (benefit)	$ (119,100,000)
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
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%				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
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 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 | RES 2018 | APS | Solar Communities
Change in regulatory asset
Program term							3 years
ACC | DSMAC 2015 | APS
Settlement Agreement
Amount of approved budget																												$ 68,900,000
Number of resource savings projects | project																													3
ACC | Power Supply Adjustor (PSA) | APS
Change in regulatory asset
Beginning balance																			$ 74,585,000		75,637,000	12,465,000	12,465,000
Deferred fuel and purchased power costs — current period																					18,950,000	988,000
Amounts refunded/(charged) to customers																					(20,002,000)	4,172,000
Ending balance																					$ 74,585,000	$ 17,625,000	$ 75,637,000
PSA rate (in dollars per kWh) | $ / kWh				0.004555				0.000555				(0.001348)
Forward component of increase in PSA (in dollars per kWh) | $ / kWh				0.002009				0.000876				(0.001027)
Historical component of increase in PSA (in dollars per kWh) | $ / kWh				0.002546				(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 | 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	$ 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
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
Minimum | ACC | RES 2018 | APS | Solar Communities
Change in regulatory asset
Required annual capital investment							$ 10,000,000
Maximum | ACC | RES 2018 | APS | Solar Communities
Change in regulatory asset
Required annual capital investment							$ 15,000,000
Subsequent Event
Change in regulatory asset
Public Utilities, Ballot Initiative, Statutory Fine Per Violation																			$ 5,000