Notes Payable Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	$ 6,567,366,000	$ 6,087,879,000
Medium-term Notes	800,000,000	700,000,000
Line of Credit Facility, Borrowing Capacity, Description	1,500,000,000
Discount Notes [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Debt Instrument, original maturity period	1 year
Due within one year	4,966,010,000	5,129,978,000
Medium-term Notes
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	194,989,000	45,000,000
Minimum [Member] | Medium-term Notes
Schedule of Short-term and Long-term Borrowings [Line Items]
Debt Instrument, original maturity period	6 months
Maximum [Member] | Discount Notes [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	$ 5,700,000,000	$ 5,100,000,000
Maximum [Member] | Medium-term Notes
Schedule of Short-term and Long-term Borrowings [Line Items]
Debt Instrument, original maturity period	15 years