Notes Payable - Borrowings (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
Short-term Debt, Weighted Average Interest Rate	0.70%	0.48%
Long-term Debt, Current Maturities	$ 2,155,784	$ 1,872,910
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 1,733,121	$ 1,200,558
Long-term Debt, Weighted Average Interest Rate	1.68%	1.59%
Long-term Debt, Excluding Current Maturities	$ 5,222,977	$ 4,967,036
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,108,854	1,177,363
Long-term Debt, Maturities, Repayments of Principal in Year Five	602,037	529,567
Long-term Debt, Maturities, Repayments of Principal in Year Four	495,096	520,284
Long-term Debt, Maturities, Repayments of Principal in Year Three	1,283,869	1,539,264
Total notes payable	$ 13,663,100	$ 14,078,497
Debt, Weighted Average Interest Rate	1.07%	0.87%
Debt Instrument, Redemption, Period Two [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.08%	1.02%
Debt Instrument, Redemption, Period Three [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.32%	0.85%
Debt Instrument, Redemption, Period Four [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.50%	1.73%
Debt Instrument, Redemption, Period Five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.83%	1.68%
Debt Instrument, Redemption, After Period Five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.01%	3.05%
Discount Notes [Member]
Debt Instrument [Line Items]
Due within one year	$ 3,789,137	$ 6,642,560
Short-term Debt, Weighted Average Interest Rate	0.59%	0.32%
Short-term Debt, Average Outstanding Amount	$ 5,753,425	$ 5,251,081
Debt Instrument, Interest Rate During Period	0.50%	0.22%
Short-term Debt, Maximum Month-end Outstanding Amount	$ 6,900,000	$ 6,700,000
Medium-term Notes [Member]
Debt Instrument [Line Items]
Due within one year	$ 2,495,202	$ 595,991
Short-term Debt, Weighted Average Interest Rate	0.70%	0.38%
Short-term Debt, Average Outstanding Amount	$ 1,551,094	$ 761,998
Debt Instrument, Interest Rate During Period	0.57%	0.25%
Long-term Debt [Member]
Debt Instrument [Line Items]
Short-term Debt, Weighted Average Interest Rate	0.90%	1.05%