Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Detail of regulatory assets
Current	$ 420,802	$ 291,713
Non-Current	1,173,977	1,133,987
Pension
Detail of regulatory assets
Current	0	0
Non-Current	496,372	469,953
Deferred fuel and purchased power
Detail of regulatory assets
Current	300,912	175,835
Non-Current	0	0
Income taxes — allowance for funds used during construction (“AFUDC”) equity
Detail of regulatory assets
Current	7,169	7,169
Non-Current	161,279	158,776
Retired power plant costs
Detail of regulatory assets
Current	28,182	28,181
Non-Current	100,123	114,214
Ocotillo deferral
Detail of regulatory assets
Current	0	0
Non-Current	124,919	95,723
SCR deferral
Detail of regulatory assets
Current	0	0
Non-Current	95,171	81,307
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	45,342	49,626
Lost fixed cost recovery
Detail of regulatory assets
Current	53,087	41,807
Non-Current	0	0
Deferred compensation
Detail of regulatory assets
Current	0	0
Non-Current	35,806	36,195
Four Corners cost deferral
Detail of regulatory assets
Current	8,077	8,077
Non-Current	20,037	24,075
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	1,113	1,113
Non-Current	23,807	24,291
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	21,174	21,255
Coal reclamation
Detail of regulatory assets
Current	1,068	1,068
Non-Current	16,465	16,999
Loss on reacquired debt
Detail of regulatory assets
Current	1,648	1,689
Non-Current	10,128	10,877
Mead-Phoenix transmission line contributions in aid of construction (“CIAC”)
Detail of regulatory assets
Current	332	332
Non-Current	9,214	9,380
Tax expense adjustor mechanism
Detail of regulatory assets
Current	7,956	6,226
Non-Current	0	0
Demand side management
Detail of regulatory assets
Current	0	0
Non-Current	7,269	7,268
Tax expense of Medicare subsidy
Detail of regulatory assets
Current	1,235	1,235
Non-Current	3,167	3,704
TCA balancing account
Detail of regulatory assets
Current	0	0
Non-Current	1,903	0
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Current	0	3,341
Non-Current	0	9,244
PSA Interest
Detail of regulatory assets
Current	133	4,355
Non-Current	0	0
Other
Detail of regulatory assets
Current	1,321	2,716
Non-Current	$ 1,801	$ 1,100