Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Detail of regulatory assets
Regulatory assets, current	$ 140,000	$ 138,000
Regulatory assets, non-current	1,102,327	1,054,087
Pension benefits
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	498,000	485,000
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Regulatory assets, current	5,000	5,000
Regulatory assets, non-current	132,000	118,000
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Regulatory assets, current	63,000	51,000
Regulatory assets, non-current	73,000	46,000
Transmission vegetation management
Detail of regulatory assets
Regulatory assets, current	7,000	9,000
Regulatory assets, non-current	0	5,000
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	6,000	7,000
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	22,000	35,000
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	36,000	34,000
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	0	7,000
Regulatory assets, non-current	0	0
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	2,000	2,000
Regulatory assets, non-current	12,000	14,000
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,000	1,000
Regulatory assets, non-current	15,000	16,000
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	2,000	2,000
Regulatory assets, non-current	49,000	46,000
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	7,000	7,000
Regulatory assets, non-current	65,000	70,000
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	43,000	38,000
Regulatory assets, non-current	0	0
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	10,000	10,000
Regulatory assets, non-current	130,000	136,000
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	46,000	30,000
Other
Detail of regulatory assets
Regulatory assets, current	0	6,000
Regulatory assets, non-current	$ 18,000	$ 12,000