Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011
Long-term debt issued
Interest rate on debt portion basis points	Variable(5)
Long term total	$ 581
Tax-Exempt Revenue Bonds [Member]
Long-term debt issued
Interest rate on debt portion expected to be repaid	2.35%
Long term total	31
Mortgage Bonds [Member]
Long-term debt issued
Interest rate on debt portion expected to be repaid	3.90%
Long term total	250
Senior Notes [Member]
Long-term debt issued
Long term total	$ 300