Credit Facilities Credit Facilities (Narrative) (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Line of Credit Facilty - Canada Lease Warehouse Canadian Dollars New	$ 800,000,000
Unamortized Debt Issuance Expense	9,800,000	4,500,000
U.S. Dollar [Member]
Line of Credit Facility [Line Items]
Line of Credit Facilities - GM Related Party Facility	5,500,000,000
Line of Credit Facilities - GM Revolving Credit Facility - sublimit	4,000,000,000
Line of Credit Facilty -Syndicated warehouse facility	$ 2,500,000,000