Debt Obligations - Line of Credit (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Line of Credit Facility [Line Items]
Line of Credit Facility, Reduction in Borrowing Capacity From Usage of Letters of Credit	$ 21
Line of Credit Facility, Maximum Borrowing Capacity	500
Line of Credit, Covenant, Maximum Total Debt to Consolidated EBITDA for Four Consecutive Quarters	3.00 to 1.00
Line of Credit Facility, Covenant, Minimum Cash Interest Coverage Ratio for Four Consecutive Fiscal Quarters	4.00 to 1.00
Line of Credit Facility, Covenant, Minimum Cash Interest Coverage Ratio for Four Consecutive Fiscal Quarters, Ratio	4
Line of Credit, Covenant, Maximum Total Debt To Consolidated EBITDA four Four Consecutive Quarters, Ratio	3
Line of Credit Facility, Amount Outstanding	$ 0	$ 0
Interest Rate Margin Charged In Addition To LIBOR	0.425%