Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Detail of regulatory assets
Regulatory assets, current	$ 209,575	$ 248,088
Regulatory assets, non-current	1,221,293	1,202,302
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	598,031	576,188
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	28,182	27,402
Regulatory assets, non-current	174,257	188,843
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Regulatory assets, current	5,882	3,828
Regulatory assets, non-current	150,684	142,852
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Regulatory assets, current	41,062	52,100
Regulatory assets, non-current	33,215	34,845
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	65,726	75,637
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	42,247	48,305
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,066	1,066
Regulatory assets, non-current	25,239	26,218
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	36,125	59,844
Regulatory assets, non-current	0	0
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	19,860	19,395
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	37,854	36,413
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	8,569	8,569
Regulatory assets, non-current	68,499	74,926
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	14,078	15,305
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,235	1,236
Regulatory assets, non-current	6,253	7,415
TCA balancing account
Detail of regulatory assets
Regulatory assets, current	7,087	1,220
Regulatory assets, non-current	0	0
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	2,654	2,654
Regulatory assets, non-current	6,482	8,472
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,127	10,376
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,546	1,068
Regulatory assets, non-current	11,695	12,396
SCR deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	16,319	353
Other
Detail of regulatory assets
Regulatory assets, current	395	3,418
Regulatory assets, non-current	$ 6,453	$ 0