Notes Payable Earliest Interest Rate Reset Date of Borrowings Outstanding (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Due within one year	$ 6,567,366	$ 6,087,879
Short-term Debt, Weighted Average Interest Rate	0.30%	0.33%
Long-term Debt, Maturities, Repayments of Principal in Year Two	1,679,364	937,690
Long-term Debt, Weighted Average Interest Rate	1.99%	2.41%
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
Total notes payable	11,602,105	10,192,761
Debt, Weighted Average Interest Rate	1.03%	1.17%
Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	63,977
Long-term Debt, Weighted Average Interest Rate	1.08%
Long-term Debt, Maturities, Repayments of Principal in Year Three	206,820
Long-term Debt, Maturities, Repayments of Principal in Year Four	33,970
Long-term Debt, Maturities, Repayments of Principal in Year Five	256,798
Long-term Debt, Maturities, Repayments of Principal after Year Five	169,533
Medium-term Notes
Debt Instrument [Line Items]
Due within one year	194,989	45,000
Short-term Debt, Weighted Average Interest Rate	0.15%	0.17%
Due year two [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	2.36%	1.11%
Due year two [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.29%
Due year three [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.40%	3.09%
Due year three [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.48%
Due year four [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.74%	2.21%
Due year four [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.59%
Due year five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.19%	2.10%
Due year five [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.91%
Due after five years [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	2.97%	3.84%
Due after five years [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	2.46%
Due after five years [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.11%
Long-term Debt, Maturities, Repayments of Principal after Year Five	577,445
Due year five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.40%
Long-term Debt, Maturities, Repayments of Principal in Year Five	359,389
Due year four [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.78%
Long-term Debt, Maturities, Repayments of Principal in Year Four	829,829
Due year three [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.90%
Long-term Debt, Maturities, Repayments of Principal in Year Three	746,592
Due within one year [Member]
Debt Instrument [Line Items]
Due within one year	7,568,453
Short-term Debt, Weighted Average Interest Rate	0.38%
Due year two [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 1,520,397
Long-term Debt, Weighted Average Interest Rate	2.58%