Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Detail of regulatory assets
Current	$ 497,900	$ 291,713
Non-Current	1,177,698	1,133,987
Pension
Detail of regulatory assets
Current	0	0
Non-Current	493,952	469,953
Deferred fuel and purchased power
Detail of regulatory assets
Current	375,181	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	164,567	158,776
Retired power plant costs
Detail of regulatory assets
Current	27,244	28,181
Non-Current	93,686	114,214
Ocotillo deferral
Detail of regulatory assets
Current	0	0
Non-Current	139,009	95,723
SCR deferral
Detail of regulatory assets
Current	0	0
Non-Current	101,890	81,307
Lost fixed cost recovery
Detail of regulatory assets
Current	58,423	41,807
Non-Current	0	0
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	43,199	49,626
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	18,018	24,075
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	1,113	1,113
Non-Current	23,185	24,291
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	21,134	21,255
Coal reclamation
Detail of regulatory assets
Current	1,068	1,068
Non-Current	16,198	16,999
Loss on reacquired debt
Detail of regulatory assets
Current	1,648	1,689
Non-Current	9,716	10,877
Mead-Phoenix transmission line contributions in aid of construction (“CIAC”)
Detail of regulatory assets
Current	332	332
Non-Current	9,131	9,380
Tax expense adjustor mechanism
Detail of regulatory assets
Current	8,614	6,226
Non-Current	0	0
PSA Interest
Detail of regulatory assets
Current	245	4,355
Non-Current	0	0
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Current	0	3,341
Non-Current	0	9,244
Demand side management
Detail of regulatory assets
Current	0	0
Non-Current	0	7,268
Other
Detail of regulatory assets
Current	217	3,951
Non-Current	$ 8,207	$ 4,804