Commitments and Contingencies - Palo Verde Nuclear Generating Station and Contractual Obligations (Details)			3 Months Ended	9 Months Ended
	Nov. 08, 2018 USD ($)	Aug. 18, 2014 USD ($)	Sep. 30, 2018 USD ($) power_plant	Sep. 30, 2018 USD ($) power_plant time_period claim	Nov. 01, 2018 power_plant	Dec. 31, 1986 Trust
Breach of Contract for Disposal of Spent Nuclear Fuel and/or High Level Radioactive Waste
Commitments and Contingencies
Litigation settlement amount		$ 57,400,000
Payments for legal settlements				$ 74,200,000
APS
Commitments and Contingencies
Maximum insurance against public liability per occurrence for a nuclear incident (up to)				13,100,000,000
Maximum available nuclear liability insurance (up to)				450,000,000
Remaining nuclear liability insurance through mandatory industry wide retrospective assessment program				12,600,000,000
Maximum retrospective premium assessment per reactor for each nuclear liability incident				127,300,000
Annual limit per incident with respect to maximum retrospective premium assessment				$ 19,000,000
Number of VIE lessor trusts			3	3		3
Maximum potential retrospective assessment per incident of APS				$ 111,100,000
Annual payment limitation with respect to maximum potential retrospective premium assessment				16,600,000
Amount of "all risk" (including nuclear hazards) insurance for property damage to, and decontamination of, property at Palo Verde				2,800,000,000
Maximum amount that APS could incur under the current NEIL policies for each retrospective assessment				24,800,000
Collateral assurance provided based on rating triggers				$ 71,200,000
Period to provide collateral assurance based on rating triggers				20 days
Purchase obligation, decrease			$ 166,000,000
APS | Breach of Contract for Disposal of Spent Nuclear Fuel and/or High Level Radioactive Waste
Commitments and Contingencies
Litigation settlement amount		$ 16,700,000
Number of claims submitted | claim				3
Number of settlement agreement time periods | time_period				3
Payments for legal settlements				$ 21,600,000
Subsequent Event | APS
Commitments and Contingencies
Maximum retrospective premium assessment per reactor for each nuclear liability incident	$ 137,600,000
Annual limit per incident with respect to maximum retrospective premium assessment	20,500,000
Number of VIE lessor trusts | power_plant					3
Maximum potential retrospective assessment per incident of APS	120,100,000
Annual payment limitation with respect to maximum potential retrospective premium assessment	$ 17,900,000