Regulatory Assets and Liabilities - Regulatory Balancing Accounts (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Regulatory balancing accounts | Energy Resource Recovery Account
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	$ 1,005	$ (135)
Regulatory balancing accounts | Four Corners Balancing Account
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	145	25
Regulatory balancing accounts | New System Generation Balancing Account
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	132	(21)
Regulatory Balancing Accounts, Liabilities | Public Purpose Programs and Energy Efficiency Programs
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	(1,037)	(994)
Regulatory Balancing Accounts, Liabilities | Base Rate Recovery Balancing Account
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	(247)	505
Regulatory Balancing Accounts, Liabilities | Greenhouse Gas Auction Revenue
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	(385)	(109)
Regulatory Balancing Accounts, Liabilities | FERC Balancing Account
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	(59)	(129)
Regulatory Balancing Accounts, Liabilities | Other Balancing Account
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	(108)	(142)
Regulatory Balancing Accounts, Liabilities | Significant Components
Regulatory Assets and Liabilities
Net Regulatory Assets Pertaining to Balancing Accounts	$ (554)	$ (1,000)