Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Detail of regulatory assets
Regulatory assets, current	$ 253,075	$ 248,088
Regulatory assets, non-current	1,200,260	1,202,302
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	569,784	576,188
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	26,668	27,402
Regulatory assets, non-current	180,298	188,843
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Regulatory assets, current	3,818	3,828
Regulatory assets, non-current	143,619	142,852
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Regulatory assets, current	62,069	52,100
Regulatory assets, non-current	45,788	34,845
Deferred fuel and purchased power (b) (d)
Detail of regulatory assets
Regulatory assets, current	74,585	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	46,285	48,305
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,066	1,066
Regulatory assets, non-current	26,198	26,218
Lost fixed cost recovery (b)
Detail of regulatory assets
Regulatory assets, current	54,384	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	19,550	19,395
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	37,650	36,413
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	8,569	8,569
Regulatory assets, non-current	73,244	74,926
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	14,896	15,305
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,235	1,236
Regulatory assets, non-current	7,387	7,415
Transmission cost adjustor
Detail of regulatory assets
Regulatory assets, current	6,867	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	7,809	8,472
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,293	10,376
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,068	1,068
Regulatory assets, non-current	12,468	12,396
Other
Detail of regulatory assets
Regulatory assets, current	46	3,418
Regulatory assets, non-current	$ 4,991	$ 353