Regulatory Matters - Schedule of Regulatory Liabilities (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Detail of regulatory liabilities
Regulatory liabilities, current	$ 154,000,000	$ 99,000,000
Regulatory liabilities, non-current	1,034,515,000	801,297,000
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	31,000,000	28,000,000
Regulatory liabilities, non-current	284,000,000	303,000,000
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	277,000,000	266,000,000
Renewable energy standard
Detail of regulatory liabilities
Regulatory liabilities, current	40,000,000	33,000,000
Regulatory liabilities, non-current	8,000,000	15,000,000
Income taxes — change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	1,000,000	0
Regulatory liabilities, non-current	72,000,000	74,000,000
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	5,000,000	6,000,000
Regulatory liabilities, non-current	53,000,000	36,000,000
Deferred gains on utility property
Detail of regulatory liabilities
Regulatory liabilities, current	2,000,000	2,000,000
Regulatory liabilities, non-current	9,000,000	10,000,000
Income taxes — deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	3,000,000	3,000,000
Regulatory liabilities, non-current	92,000,000	79,000,000
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	39,000,000	27,000,000
Regulatory liabilities, non-current	0	0
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	33,000,000	0
Regulatory liabilities, non-current	221,000,000	0
Other
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	$ 19,000,000	$ 18,000,000