Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Detail of regulatory assets
Regulatory assets, current	$ 166,902	$ 248,088
Regulatory assets, non-current	1,342,941	1,202,302
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	733,351	576,188
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	28,182	27,402
Regulatory assets, non-current	167,164	188,843
Income taxes - AFUDC equity
Detail of regulatory assets
Regulatory assets, current	6,457	3,828
Regulatory assets, non-current	151,467	142,852
Deferred fuel and purchased power - mark-to-market
Detail of regulatory assets
Regulatory assets, current	31,728	52,100
Regulatory assets, non-current	23,768	34,845
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	37,164	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	40,228	48,305
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,079	1,066
Regulatory assets, non-current	25,522	26,218
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	32,435	59,844
Regulatory assets, non-current	0	0
Palo Verde VIE
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	20,015	19,395
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	36,523	36,413
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	8,569	8,569
Regulatory assets, non-current	66,356	74,926
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	13,668	15,305
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,235	1,236
Regulatory assets, non-current	6,176	7,415
TCA balancing account
Detail of regulatory assets
Regulatory assets, current	3,860	1,220
Regulatory assets, non-current	772	0
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	2,654	2,654
Regulatory assets, non-current	5,819	8,472
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,044	10,376
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,546	1,068
Regulatory assets, non-current	15,607	12,396
SCR deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	23,276	353
Other
Detail of regulatory assets
Regulatory assets, current	1,947	3,418
Regulatory assets, non-current	$ 3,185	$ 0