Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Detail of regulatory assets
Current	$ 486,260	$ 518,524
Non-Current	1,184,246	1,192,987
Pension
Detail of regulatory assets
Current	0	0
Non-Current	506,280	509,751
Deferred fuel and purchased power
Detail of regulatory assets
Current	354,816	388,148
Non-Current	0	0
Income taxes — allowance for funds used during construction (“AFUDC”) equity
Detail of regulatory assets
Current	7,625	7,625
Non-Current	165,071	164,768
Ocotillo deferral
Detail of regulatory assets
Current	9,507	9,507
Non-Current	135,766	138,143
Retired power plant costs
Detail of regulatory assets
Current	15,455	15,160
Non-Current	94,902	99,681
SCR deferral
Detail of regulatory assets
Current	8,147	8,147
Non-Current	95,588	97,624
Lost fixed cost recovery
Detail of regulatory assets
Current	57,808	63,889
Non-Current	0	0
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	38,915	41,057
Deferred compensation
Detail of regulatory assets
Current	0	0
Non-Current	35,355	33,997
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	826	1,129
Non-Current	23,899	23,639
Four Corners cost deferral
Detail of regulatory assets
Current	8,077	8,077
Non-Current	13,979	15,998
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	21,053	21,094
Coal reclamation
Detail of regulatory assets
Current	2,978	2,978
Non-Current	13,118	13,862
Loss on reacquired debt
Detail of regulatory assets
Current	1,648	1,648
Non-Current	8,976	9,372
Active Union Medical Trust
Detail of regulatory assets
Current	0	0
Non-Current	10,453	1,175
TCA balancing account
Detail of regulatory assets
Current	8,205	170
Non-Current	2,038	3,663
Mead-Phoenix transmission line contributions in aid of construction (“CIAC”)
Detail of regulatory assets
Current	332	332
Non-Current	8,965	9,048
Tax expense adjustor mechanism
Detail of regulatory assets
Current	656	656
Non-Current	5,681	5,845
Tax expense of Medicare subsidy
Detail of regulatory assets
Current	1,235	1,235
Non-Current	2,406	2,469
Other
Detail of regulatory assets
Current	376	1,254
Non-Current	$ 1,801	$ 1,801