Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Detail of regulatory assets
Current	$ 251,594	$ 203,207
Non-Current	1,315,846	1,304,073
Pension
Detail of regulatory assets
Current	0	0
Non-Current	663,327	660,223
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Current	6,815	6,800
Non-Current	156,528	154,974
Retired power plant costs
Detail of regulatory assets
Current	28,182	28,182
Non-Current	128,304	142,503
Deferred fuel and purchased power
Detail of regulatory assets
Current	101,425	70,137
Non-Current	0	0
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Current	42,911	36,887
Non-Current	26,151	33,185
Ocotillo deferral
Detail of regulatory assets
Current	0	0
Non-Current	65,571	38,144
SCR deferral
Detail of regulatory assets
Current	0	0
Non-Current	64,971	52,644
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	53,911	58,196
Deferred compensation
Detail of regulatory assets
Current	0	0
Non-Current	36,481	36,464
Four Corners cost deferral
Detail of regulatory assets
Current	8,077	8,077
Non-Current	28,113	32,152
Lost fixed cost recovery
Detail of regulatory assets
Current	34,144	26,067
Non-Current	0	0
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	1,098	1,098
Non-Current	24,532	24,981
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	20,945	20,635
Coal reclamation
Detail of regulatory assets
Current	1,068	1,546
Non-Current	17,533	17,688
Loss on reacquired debt
Detail of regulatory assets
Current	1,637	1,637
Non-Current	11,241	12,031
TCA balancing account
Detail of regulatory assets
Current	8,272	6,324
Non-Current	2,926	2,885
Mead-Phoenix transmission line contributions in aid of construction (CIAC)
Detail of regulatory assets
Current	332	332
Non-Current	9,546	9,712
Tax expense of Medicare subsidy
Detail of regulatory assets
Current	1,238	1,235
Non-Current	4,444	4,940
AG-1 deferral
Detail of regulatory assets
Current	2,787	2,787
Non-Current	1,322	2,716
Tax expense adjustor mechanism
Detail of regulatory assets
Current	3,640	1,612
Non-Current	0	0
Other
Detail of regulatory assets
Current	1,399	1,917
Non-Current	$ 0	$ 0