Regulatory Matters - Retail Rate Case Filing (Details) - Retail Rate Case Filing with Arizona Corporation Commission - ACC - APS	Jan. 03, 2018 Customer	Nov. 13, 2017 USD ($)	Mar. 27, 2017 USD ($) $ / kWh	Mar. 26, 2017 $ / kWh	Jun. 01, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 01, 2011 USD ($)	Sep. 30, 2018 appeal
Public Utilities, General Disclosures [Line Items]
Net retail rate increase					$ 165,900,000		$ 95,500,000
Adjustor account balance transferred into base rates, amount						$ 267,600,000
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%
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			$ 61,000,000.0
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
Periodic metering infrastructure opt-out fee		$ 5
Number of appeals | appeal								2
Number of customers named in complaint | Customer	25
AZ Sun Program Phase 2
Public Utilities, General Disclosures [Line Items]
Public utilities, minimum annual renewable energy standard and tariff			$ 10,000,000
Public utilities, maximum annual renewable energy standard and tariff			$ 15,000,000