Basis of Presentation (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 item	Jun. 30, 2012
Basis of Presentation
Number of states in which service is provided	14
Out-of-Period Adjustment
Net cash provided by operating activities	$ 1,298	$ 1,306
Net cash used in investing activities	(544)	(1,624)
Net cash provided by financing activities	(754)	319
Correction of presentation of certain transactions with affiliates in consolidated statements of cash flows | As Reported
Out-of-Period Adjustment
Net cash provided by operating activities		1,788
Net cash used in investing activities		(1,960)
Net cash provided by financing activities		173
Correction of presentation of certain transactions with affiliates in consolidated statements of cash flows | Error Correction
Out-of-Period Adjustment
Net cash provided by operating activities		(482)
Net cash used in investing activities		336
Net cash provided by financing activities		$ 146