Debt (Letter of Credit) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	$ 603	$ 626
Corporate Revolving Facility [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	222	243
CDH [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	250	253
Various Project Financing Facilities [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	$ 131	$ 130