Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Detail of regulatory assets
Regulatory assets, current	$ 248,088	$ 106,875
Regulatory assets, non-current	1,202,302	1,313,428
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	576,188	711,059
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	27,402	9,913
Regulatory assets, non-current	188,843	117,591
Income taxes - AFUDC equity
Detail of regulatory assets
Regulatory assets, current	3,828	6,305
Regulatory assets, non-current	142,852	152,118
Deferred fuel and purchased power - mark-to-market
Detail of regulatory assets
Regulatory assets, current	52,100	0
Regulatory assets, non-current	34,845	42,963
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	8,077	6,689
Regulatory assets, non-current	48,305	56,894
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	1,066	2,120
Regulatory assets, non-current	26,218	54,356
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	59,844	61,307
Regulatory assets, non-current	0	0
Palo Verde VIE
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	19,395	18,775
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	36,413	35,595
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	8,569	0
Regulatory assets, non-current	74,926	73,200
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	15,305	16,942
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	2,654	0
Regulatory assets, non-current	8,472	5,868
Demand side management
Detail of regulatory assets
Regulatory assets, current	0	3,744
Regulatory assets, non-current	0	0
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,236	1,513
Regulatory assets, non-current	7,415	10,589
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,376	10,708
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	75,637	12,465
Regulatory assets, non-current	0	0
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	1,068	418
Regulatory assets, non-current	12,396	5,182
Other
Detail of regulatory assets
Regulatory assets, current	4,638	432
Regulatory assets, non-current	$ 353	$ 1,588