Regulatory Matters - Schedule of Regulatory Liabilities (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Detail of regulatory liabilities
Regulatory liabilities, current	$ 129,000,000	$ 131,000,000
Regulatory liabilities, non-current	1,070,106,000	1,051,196,000
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	37,000,000	31,000,000
Regulatory liabilities, non-current	264,000,000	273,000,000
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	302,000,000	296,000,000
Renewable energy standard
Detail of regulatory liabilities
Regulatory liabilities, current	25,000,000	25,000,000
Regulatory liabilities, non-current	25,000,000	23,000,000
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	1,000,000	0
Regulatory liabilities, non-current	71,000,000	72,000,000
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	5,000,000	5,000,000
Regulatory liabilities, non-current	66,000,000	66,000,000
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,000,000	2,000,000
Regulatory liabilities, non-current	8,000,000	8,000,000
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	3,000,000	4,000,000
Regulatory liabilities, non-current	93,000,000	93,000,000
Deferred fuel and purchased power
Detail of regulatory liabilities
Regulatory liabilities, current	16,000,000	0
Regulatory liabilities, non-current	0	0
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	5,000,000	31,000,000
Regulatory liabilities, non-current	27,000,000	0
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	32,000,000	32,000,000
Regulatory liabilities, non-current	191,000,000	199,000,000
Other
Detail of regulatory liabilities
Regulatory liabilities, current	3,000,000	1,000,000
Regulatory liabilities, non-current	$ 23,000,000	$ 21,000,000