Debt (Details) (USD $) In Millions	Jun. 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 June 30, 2011 and 2.2 % December 31, 2010)	144	212
Senior notes and debentures, maturing through 2039, interest rates ranging from 4.60% to 7.75% (weighted average interest rate of 6.3% at June 30, 2011 and 6.5% at December 31,2010)	5,710	5,452
Tax-exempt bonds maturing through 2039, fixed and variable interest rates ranging from 0.1% to 7.4% (weighted average interest rate of 3.1% at June 30, 2011 and December 31, 2010)	2,671	2,696
Tax-exempt project bonds, principal payable in periodic installments, maturing through 2029, fixed and variable interest rates ranging from 0.1% to 3.4% (weighted average interest rate of 1.3% at June 30, 2011 and 2.5% at December 31, 2010)	86	116
Capital leases and other, maturing through 2050, interest rates up to 12%	426	431
Components of Debt, total	9,037	8,907
Current portion of long-term debt	198	233
Long term Debt, less current portion of long-term debt	$ 8,839	$ 8,674