Debt (Details Textual) (USD $)	3 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended			9 Months Ended		9 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
	Mar. 31, 2011	Sep. 30, 2011	Feb. 28, 2011 4.60% senior notes due March 2021 [Member]	Mar. 31, 2011 7.65% senior notes [Member]	Aug. 31, 2011 2.60% Senior Notes due September 2016 [Member]	Sep. 30, 2011 Capital leases and other [Member]	Sep. 30, 2011 Senior notes and debentures [Member]	Dec. 31, 2010 Senior notes and debentures [Member]	Sep. 30, 2011 Tax exempt project bonds [Member]	Dec. 31, 2010 Tax exempt project bonds [Member]	Sep. 30, 2011 Tax exempt bonds [Member]	Dec. 31, 2010 Tax exempt bonds [Member]	Sep. 30, 2011 Canadian credit facility [Member]	Dec. 31, 2010 Canadian credit facility [Member]	Sep. 30, 2011 Revolving Credit Facility [Member]	Sep. 30, 2011 Revolving Credit Facility [Member]	Sep. 30, 2011 Letter of Credit [Member]
Debt (Textual) [Abstract]
Weighted average interest rate							6.00%	6.50%	1.40%	2.50%	3.00%	3.10%
Interest rate upper range						0.12	0.0775		0.034		0.074
Interest rate lower range							0.0260		0.002		0.002
Issuance of senior notes			$ 400,000,000		$ 500,000,000
Repayment of senior notes matured				147,000,000
Repayment under revolving credit facility															100,000,000
Borrowing under revolving credit facility															100,000,000
Interest rate of Senior notes			4.60%	7.65%	2.60%
Debt maturing within twelve months, total		348,000,000											133,000,000
Start period of maturity for debt instruments																	2013
End period of maturity for debt instruments						2050	2039		2029		2041					2016	2015
Weighted average effective interest rate													2.30%	2.20%
Net Repayment of Canadian credit facility debt													77,000,000
Revolving credit facility, total capacity															2,000,000,000	2,000,000,000
Debt (Additional) (Textual) [Abstract]
Debt maturing within 12 months classified as long-term		123,000,000
Issuance of tax exempt bonds		80,000,000
Repayment of tax-exempt bonds		25,000,000
Repayment of tax-exempt project bonds		30,000,000
Letter of credit outstanding, revolving credit and letter of credit facilities		1,500,000,000
Proceeds from the debt issuance	396,000,000	497,000,000
Revolving credit and credit facilities, aggregate capacity		2,500,000,000
Letter of credit outstanding, revolving credit facility		$ 1,000,000