Notes Payable - Earliest Interest Reset Date of Borrowing Outstanding (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Due within one year	$ 8,440,123	$ 9,111,461
Long-term Debt, Maturities, Repayments of Principal in Year Two	1,733,121	1,200,558
Long-term Debt, Maturities, Repayments of Principal in Year Three	1,283,869	1,539,264
Long-term Debt, Maturities, Repayments of Principal in Year Four	495,096	520,284
Long-term Debt, Maturities, Repayments of Principal in Year Five	602,037	529,567
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,108,854	1,177,363
Total notes payable	$ 13,663,100	$ 14,078,497
Short-term Debt, Weighted Average Interest Rate	0.70%	0.48%
Long-term Debt, Weighted Average Interest Rate	1.68%	1.59%
Debt, Weighted Average Interest Rate	1.07%	0.87%
Meduim Term Notes Called During the Period	$ 1,300,000	$ 2,400,000
Due within one year [Member]
Debt Instrument [Line Items]
Due within one year	$ 9,416,805
Short-term Debt, Weighted Average Interest Rate	0.73%
Due year two [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 1,488,131
Long-term Debt, Weighted Average Interest Rate	1.12%
Due year three [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 919,021
Long-term Debt, Weighted Average Interest Rate	1.47%
Due year four [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Four	$ 329,181
Long-term Debt, Weighted Average Interest Rate	1.77%
Due year five [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Five	$ 547,078
Long-term Debt, Weighted Average Interest Rate	1.91%
Due after five years [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 962,884
Long-term Debt, Weighted Average Interest Rate	3.27%