Commitments and Contingencies - Environmental Matters and Financial Assurances (Details) - USD ($) $ in Thousands				3 Months Ended
	Feb. 22, 2021	Jul. 03, 2018	Jul. 06, 2016	Mar. 31, 2021
Financial Assurances
Production tax credit guarantees				$ 2,000
Equity contribution guarantees				38,000
APS | 2017 Settlement Agreement and its Customer Education and Outreach Plan
Arizona Attorney General [Abstract]
Settlement amount	$ 24,750
Settlement amount returned to customers	$ 24,000
APS | Letters of Credit Expiring in 2020
Financial Assurances
Outstanding letters of credit				5,200
APS | Surety Bonds Expiring in 2020
Financial Assurances
Surety bonds expiring, amount				16,000
4C Acquisition, LLC | Four Corners
Environmental Matters [Abstract]
Percentage of share of cost of control			7.00%
Four Corners Coal Supply Agreement
Notes receivable, related parties				23,000
4C Acquisition, LLC | Coal Supply Agreement Arbitration | Four Corners
Four Corners Coal Supply Agreement
Reimbursement payments due to 4CA				$ 10,000
NTEC | Four Corners
Four Corners Coal Supply Agreement
Option to purchase ownership interest (as a percent)		7.00%	7.00%
Proceeds from operating and maintenance cost reimbursement		$ 70,000
NTEC | Coal Supply Agreement Arbitration | Four Corners
Four Corners Coal Supply Agreement
Option to purchase ownership interest (as a percent)			7.00%
Regional Haze Rules | APS | Four Corners Units 4 and 5
Environmental Matters [Abstract]
Percentage of share of cost of control				63.00%
Expected environmental cost				$ 400,000
Regional Haze Rules | APS | Natural gas tolling contract obligations | Four Corners Units 4 and 5
Environmental Matters [Abstract]
Additional percentage share of cost of control				7.00%
Regional Haze Rules | APS | Four Corners | Four Corners Units 4 and 5
Environmental Matters [Abstract]
Site contingency increase in loss exposure not accrued, best estimate				$ 45,000
Coal combustion waste | APS | Four Corners
Environmental Matters [Abstract]
Site contingency increase in loss exposure not accrued, best estimate				27,000
Coal combustion waste | APS | Navajo Plant
Environmental Matters [Abstract]
Site contingency increase in loss exposure not accrued, best estimate				1,000
Minimum | Coal combustion waste | APS | Cholla
Environmental Matters [Abstract]
Site contingency increase in loss exposure not accrued, best estimate				16,000
Minimum | Coal combustion waste | APS | Cholla and Four Corners
Environmental Matters [Abstract]
Site contingency increase in loss exposure not accrued, best estimate				10,000
Maximum | Coal combustion waste | APS | Cholla and Four Corners
Environmental Matters [Abstract]
Site contingency increase in loss exposure not accrued, best estimate				$ 15,000