Regulatory Matters - Schedule of Regulatory Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Detail of regulatory liabilities
Regulatory liabilities, current	$ 131,000,000	$ 99,000,000
Regulatory liabilities, non-current	1,051,196,000	801,297,000
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	31,000,000	28,000,000
Regulatory liabilities, non-current	273,000,000	303,000,000
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, current	0	0
Regulatory liabilities, non-current	296,000,000	266,000,000
Renewable energy standard
Detail of regulatory liabilities
Regulatory liabilities, current	25,000,000	33,000,000
Regulatory liabilities, non-current	23,000,000	15,000,000
Income taxes - change in rates
Detail of regulatory liabilities
Regulatory liabilities, current	0	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	66,000,000	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	8,000,000	10,000,000
Income taxes-deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	4,000,000	3,000,000
Regulatory liabilities, non-current	93,000,000	79,000,000
Demand side management
Detail of regulatory liabilities
Regulatory liabilities, current	31,000,000	27,000,000
Regulatory liabilities, non-current	0	0
Other postretirement benefits
Detail of regulatory liabilities
Regulatory liabilities, current	32,000,000	0
Regulatory liabilities, non-current	199,000,000	0
Other
Detail of regulatory liabilities
Regulatory liabilities, current	1,000,000	0
Regulatory liabilities, non-current	$ 21,000,000	$ 18,000,000