Regulatory Matters - Cost Recovery Mechanism and Net Metering (Details)																		3 Months Ended		12 Months Ended
	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	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) $ / kWh	Dec. 31, 2012 $ / kWh	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																		$ (16,709,000)	$ 18,950,000
Amounts charged to customers																		(12,872,000)	(20,002,000)
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																		(16,709,000)	18,950,000
Amounts charged to customers																		$ (12,872,000)	(20,002,000)
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
Increase (decrease) in amount of adjustment representing prorated sales losses		$ (24,500,000)						$ (3,000,000)
Amount of adjustment representing prorated sales losses pending approval		$ 36,200,000						$ 60,700,000
ACC | APS
Change in regulatory asset
Gross-up for revenue requirement of rate regulation				$ 86,500,000						$ 119,100,000
ACC | RES | APS
Settlement Agreement
Plan term																		5 years
ACC | RES 2018 | APS
Settlement Agreement
Amount of proposed budget																							$ 89,900,000			$ 90,000,000
ACC | RES 2018 | APS | Solar Communities
Change in regulatory asset
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
Deferred fuel and purchased power costs — current period																		(16,709,000)	18,950,000
Amounts charged to customers																		(12,872,000)	(20,002,000)
Ending balance																		7,583,000	$ 74,585,000	$ 75,637,000
PSA rate (in dollars per kWh) | $ / kWh			0.001658						0.004555			0.000555		(0.001348)
Forward component of increase in PSA (in dollars per kWh) | $ / kWh			0.000536						0.002009			0.000876		(0.001027)
Historical 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					$ (22,700,000)								$ 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			(0.002897)														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 | ACC | APS
Change in regulatory asset
Gross-up for revenue requirement of rate regulation	$ 34,500,000
Deferred taxes amortization, period	28 years 6 months