Regulatory Matters (Details 3) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Detail of regulatory liabilities
Regulatory liabilities, current	$ 77,000,000	$ 88,000,000
Regulatory liabilities, non-current	805,230,000	759,201,000
Removal costs
Detail of regulatory liabilities
Regulatory liabilities, current	28,000,000	27,000,000
Regulatory liabilities, non-current	320,000,000	321,000,000
Asset retirement obligations
Detail of regulatory liabilities
Regulatory liabilities, non-current	273,000,000	256,000,000
Renewable energy standard
Detail of regulatory liabilities
Regulatory liabilities, current	29,000,000	43,000,000
Regulatory liabilities, non-current	16,000,000
Income taxes - change in rates
Detail of regulatory liabilities
Regulatory liabilities, non-current	66,000,000	66,000,000
Spent nuclear fuel
Detail of regulatory liabilities
Regulatory liabilities, current	5,000,000	10,000,000
Regulatory liabilities, non-current	38,000,000	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	12,000,000	12,000,000
Income taxes-deferred investment tax credit
Detail of regulatory liabilities
Regulatory liabilities, current	2,000,000	2,000,000
Regulatory liabilities, non-current	63,000,000	52,000,000
Other
Detail of regulatory liabilities
Regulatory liabilities, current	11,000,000	4,000,000
Regulatory liabilities, non-current	$ 17,000,000	$ 16,000,000