Regulatory Matters - Schedule of Regulatory Assets (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Detail of regulatory assets
Regulatory assets, current	$ 138,000,000	$ 97,000,000
Regulatory assets, non-current	1,054,087,000	711,712,000
Pension and other postretirement benefits
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	485,000,000	314,000,000
Income taxes - AFUDC equity
Detail of regulatory assets
Regulatory assets, current	5,000,000	4,000,000
Regulatory assets, non-current	118,000,000	105,000,000
Deferred fuel and purchased power - mark-to-market
Detail of regulatory assets
Regulatory assets, current	51,000,000	5,000,000
Regulatory assets, non-current	46,000,000	29,000,000
Transmission vegetation management
Detail of regulatory assets
Regulatory assets, current	9,000,000	9,000,000
Regulatory assets, non-current	5,000,000	14,000,000
Coal reclamation
Detail of regulatory assets
Regulatory assets, current	0	8,000,000
Regulatory assets, non-current	7,000,000	18,000,000
Palo Verde VIE
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	35,000,000	41,000,000
Deferred compensation
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	34,000,000	34,000,000
Deferred fuel and purchased power
Detail of regulatory assets
Regulatory assets, current	7,000,000	21,000,000
Regulatory assets, non-current	0	0
Tax expense of Medicare subsidy
Detail of regulatory assets
Regulatory assets, current	2,000,000	2,000,000
Regulatory assets, non-current	14,000,000	15,000,000
Loss on reacquired debt
Detail of regulatory assets
Regulatory assets, current	1,000,000	1,000,000
Regulatory assets, non-current	16,000,000	17,000,000
Income taxes - investment tax credit basis adjustment
Detail of regulatory assets
Regulatory assets, current	2,000,000	1,000,000
Regulatory assets, non-current	46,000,000	39,000,000
Pension and other postretirement benefits deferral
Detail of regulatory assets
Regulatory assets, current	4,000,000	8,000,000
Regulatory assets, non-current	0	4,000,000
Four Corners cost deferral
Detail of regulatory assets
Regulatory assets, current	7,000,000	0
Regulatory assets, non-current	70,000,000	37,000,000
Lost fixed cost recovery
Detail of regulatory assets
Regulatory assets, current	38,000,000	25,000,000
Regulatory assets, non-current	0	0
Transmission cost adjustor
Detail of regulatory assets
Regulatory assets, current	0	8,000,000
Regulatory assets, non-current	0	2,000,000
Retired power plant costs
Detail of regulatory assets
Regulatory assets, current	10,000,000	3,000,000
Regulatory assets, non-current	136,000,000	18,000,000
Deferred property taxes
Detail of regulatory assets
Regulatory assets, current	0	0
Regulatory assets, non-current	30,000,000	11,000,000
Other
Detail of regulatory assets
Regulatory assets, current	2,000,000	2,000,000
Regulatory assets, non-current	$ 12,000,000	$ 14,000,000