Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Detail of regulatory assets
Current	$ 339,880	$ 291,713
Non-Current	1,135,857	1,133,987
Pension
Detail of regulatory assets
Current	0	0
Non-Current	467,423	469,953
Deferred fuel and purchased power
Detail of regulatory assets
Current	228,609	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	159,119	158,776
Retired power plant costs
Detail of regulatory assets
Current	28,182	28,181
Non-Current	107,169	114,214
Ocotillo deferral
Detail of regulatory assets
Current	0	0
Non-Current	110,820	95,723
SCR deferral
Detail of regulatory assets
Current	0	0
Non-Current	88,044	81,307
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	47,484	49,626
Lost fixed cost recovery
Detail of regulatory assets
Current	45,905	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	22,056	24,075
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	1,113	1,113
Non-Current	24,221	24,291
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	21,409	21,255
Coal reclamation
Detail of regulatory assets
Current	1,068	1,068
Non-Current	16,732	16,999
Loss on reacquired debt
Detail of regulatory assets
Current	1,703	1,689
Non-Current	10,486	10,877
Mead-Phoenix transmission line contributions in aid of construction (“CIAC”)
Detail of regulatory assets
Current	332	332
Non-Current	9,297	9,380
Demand side management
Detail of regulatory assets
Current	0	0
Non-Current	7,268	7,268
Tax expense adjustor mechanism
Detail of regulatory assets
Current	5,854	6,226
Non-Current	0	0
Tax expense of Medicare subsidy
Detail of regulatory assets
Current	1,235	1,235
Non-Current	3,626	3,704
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Current	0	3,341
Non-Current	3,728	9,244
PSA Interest
Detail of regulatory assets
Current	46	4,355
Non-Current	0	0
Other
Detail of regulatory assets
Current	2,018	2,716
Non-Current	$ 1,169	$ 1,100