Notes Payable Borrowings (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Due within one year [Abstract]
Short-term Debt	$ 6,567,366	$ 6,087,879
Short-term Debt, Weighted Average Interest Rate	0.30%	0.33%
Long-term Debt, Current Maturities	1,406,367	912,901
Long-term Debt, Fiscal Year Maturity [Abstract]
Long-term Debt, Weighted Average Interest Rate	1.99%	2.41%
Long-term Debt, Maturities, Repayments of Principal in Year Two	1,679,364	937,690
Long-term Debt, Maturities, Repayments of Principal in Year Three	1,128,413	1,277,938
Long-term Debt, Maturities, Repayments of Principal in Year Four	863,799	632,905
Long-term Debt, Maturities, Repayments of Principal in Year Five	616,187	760,772
Long-term Debt, Maturities, Repayments of Principal after Year Five	746,976	495,577
Long-term Debt, Excluding Current Maturities	5,034,739	4,104,882
Total notes payable	11,602,105	10,192,761
Debt, Weighted Average Interest Rate	1.03%	1.17%
Discount Notes [Member]
Due within one year [Abstract]
Short-term Debt	4,966,010	5,129,978
Short-term Debt, Weighted Average Interest Rate	0.16%	0.14%
Short-term Debt, Average Outstanding Amount	5,181,559	4,102,109
Debt Instrument, Interest Rate During Period	0.18%	0.22%
Medium-term Notes
Due within one year [Abstract]
Short-term Debt	194,989	45,000
Short-term Debt, Weighted Average Interest Rate	0.15%	0.17%
Short-term Debt, Average Outstanding Amount	$ 84,961	$ 130,009
Debt Instrument, Interest Rate During Period	0.20%	0.30%
Long-term Debt [Member]
Due within one year [Abstract]
Short-term Debt, Weighted Average Interest Rate	0.80%	1.40%
Due year two [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Long-term Debt, Weighted Average Interest Rate	2.36%	1.11%
Due year three [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Long-term Debt, Weighted Average Interest Rate	1.40%	3.09%
Due year four [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Long-term Debt, Weighted Average Interest Rate	1.74%	2.21%
Due year five [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Long-term Debt, Weighted Average Interest Rate	1.19%	2.10%
Due after five years [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Long-term Debt, Weighted Average Interest Rate	2.97%	3.84%

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