Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Detail of regulatory assets
Regulatory assets, current	$ 155,941	$ 106,875
Regulatory assets, non-current	1,321,473	1,313,428
Pension
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	699,817	711,059
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	9,913	9,913
Regulatory assets, non-current	115,110	117,591
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Regulatory assets, current	6,202	6,305
Regulatory assets, non-current	150,629	152,118
Deferred fuel and purchased power — mark-to-market (Note 6)
Detail of regulatory assets
Regulatory assets, current	30,203	0
Regulatory assets, non-current	59,428	42,963
Deferred fuel and purchased power (b) (e)
Detail of regulatory assets
Regulatory assets, current	17,625	12,465
Regulatory assets, non-current	0	0
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	6,689	6,689
Regulatory assets, non-current	55,221	56,894
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	2,120	2,120
Regulatory assets, non-current	54,265	54,356
Lost fixed cost recovery (b)
Detail of regulatory assets
Regulatory assets, current	70,762	61,307
Regulatory assets, non-current	0	0
Palo Verde VIEs (Note 5)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	18,930	18,775
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	36,846	35,595
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	79,447	73,200
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,637	1,637
Regulatory assets, non-current	16,533	16,942
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	1,503	1,513
Regulatory assets, non-current	10,458	10,589
Demand side management (b)
Detail of regulatory assets
Regulatory assets, current	5,491	3,744
Regulatory assets, non-current	0	0
AG-1 deferral
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	6,976	5,868
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Regulatory assets, current	332	332
Regulatory assets, non-current	10,625	10,708
Transmission cost adjustor (b)
Detail of regulatory assets
Regulatory assets, current	2,071	0
Regulatory assets, non-current	2,460	1,588
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	418	418
Regulatory assets, non-current	4,728	5,182
Other
Detail of regulatory assets
Regulatory assets, current	975	432
Regulatory assets, non-current	$ 0	$ 0