Debt (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Components of Debt
Revolving credit facility	$ 0	$ 0
Letter of credit facilities	0	0
Canadian credit facility (weighted average effective interest rate of 2.3 % at September 30, 2011 and 2.2 % December 31, 2010)	133	212
Senior notes and debentures, maturing through 2039, interest rates ranging from 2.60% to 7.75% (weighted average interest rate of 6.0% at September 30, 2011 and 6.5% at December 31,2010)	6,233	5,452
Tax-exempt bonds maturing through 2041, fixed and variable interest rates ranging from 0.2% to 7.4% (weighted average interest rate of 3.0% at September 30, 2011 and 3.1% at December 31, 2010)	2,751	2,696
Tax-exempt project bonds, maturing through 2029, fixed and variable interest rates ranging from 0.2% to 3.4% (weighted average interest rate of 1.4% at September 30, 2011 and 2.5% at December 31, 2010)	86	116
Capital leases and other, maturing through 2050, interest rates up to 12%	410	431
Components of Debt, total	9,613	8,907
Current portion of long-term debt	225	233
Long term Debt, less current portion of long-term debt	$ 9,388	$ 8,674