Regulatory Assets and Liabilities (Regulatory Balancing Accounts) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Regulatory balancing accounts | Energy resource recovery account
Regulatory Assets and Liabilities
Net liability	$ 1,478	$ 1,005
Regulatory balancing accounts | Four Corners memorandum account
Regulatory Assets and Liabilities
Net liability	4	145
Regulatory balancing accounts | New system generation balancing account
Regulatory Assets and Liabilities
Net liability	99	132
Refunds of excess revenue | Significant Components
Regulatory Assets and Liabilities
Net liability	(62)	(554)
Refunds of excess revenue | Public purpose programs and energy efficiency programs
Regulatory Assets and Liabilities
Net liability	(959)	(1,037)
Refunds of excess revenue | Base rate recovery balancing account
Regulatory Assets and Liabilities
Net liability	(29)	(247)
Refunds of excess revenue | Greenhouse gas auction revenue
Regulatory Assets and Liabilities
Net liability	(470)	(385)
Refunds of excess revenue | FERC balancing accounts
Regulatory Assets and Liabilities
Net liability	(13)	(59)
Refunds of excess revenue | Other
Regulatory Assets and Liabilities
Net liability	$ (172)	$ (108)