Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Detail of regulatory assets
Regulatory assets, current	$ 135,225	$ 166,902
Regulatory assets, non-current	1,321,507	1,342,941
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	723,307	733,351
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	28,182	28,182
Regulatory assets, non-current	160,167	167,164
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Regulatory assets, current	6,457	6,457
Regulatory assets, non-current	151,541	151,467
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Regulatory assets, current	29,340	31,728
Regulatory assets, non-current	14,360	23,768
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	7,583	37,164
Regulatory assets, non-current	0	0
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	8,077	8,077
Regulatory assets, non-current	38,209	40,228
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,079	1,079
Regulatory assets, non-current	25,475	25,522
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	29,698	32,435
Regulatory assets, non-current	0	0
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	20,170	20,015
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	37,581	36,523
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	8,569	8,569
Regulatory assets, non-current	64,214	66,356
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	13,259	13,668
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,235	1,235
Regulatory assets, non-current	6,122	6,176
TCA balancing account
Detail of regulatory assets
Regulatory assets, current	306	3,860
Regulatory assets, non-current	0	772
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	2,654	2,654
Regulatory assets, non-current	5,155	5,819
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	9,961	10,044
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,546	1,546
Regulatory assets, non-current	17,392	15,607
SCR deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	30,581	23,276
Tax expense adjustor mechanism
Detail of regulatory assets
Regulatory assets, current	5,451	0
Regulatory assets, non-current	0	0
Other
Detail of regulatory assets
Regulatory assets, current	3,079	1,947
Regulatory assets, non-current	$ 4,013	$ 3,185