Debt (Fair Values) (Details) (Fair Value, Inputs, Level 2 [Member], USD $) In Billions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Aggregate fair value of debt	$ 17.4	$ 17.6
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Aggregate fair value of debt	$ 14.9	$ 13.9