Notes Payable - Borrowings and Callable Debt(Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	$ 9,111,461	$ 7,353,953
Short-term Debt, Weighted Average Interest Rate	0.48%	0.32%
Long-term Debt, Current Maturities	$ 1,872,910	$ 1,673,137
Long-term Debt, Maturities, Repayments of Principal in Year Two	1,200,558	1,569,961
Long-term Debt, Maturities, Repayments of Principal in Year Three	1,539,264	1,219,192
Long-term Debt, Maturities, Repayments of Principal in Year Four	520,284	682,224
Long-term Debt, Maturities, Repayments of Principal in Year Five	529,567	872,576
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 1,177,363	$ 1,127,233
Long-term Debt, Weighted Average Interest Rate	1.59%	1.56%
Long-term Debt, Excluding Current Maturities	$ 4,967,036	$ 5,471,186
Debt and Capital Lease Obligations	$ 14,078,497	$ 12,825,139
Debt, Weighted Average Interest Rate	0.87%	0.85%
Due after one year	$ 4,967,036	$ 5,471,186
Discount Notes [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	$ 6,642,560	$ 4,925,828
Short-term Debt, Weighted Average Interest Rate	0.32%	0.11%
Short-term Debt, Average Outstanding Amount	$ 5,251,081	$ 4,059,708
Debt Instrument, Interest Rate During Period	0.22%	0.15%
Short-term Debt, Maximum Month-end Outstanding Amount	$ 6,700,000	$ 4,900,000
Medium-term Notes [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	$ 595,991	$ 754,988
Short-term Debt, Weighted Average Interest Rate	0.38%	0.17%
Short-term Debt, Average Outstanding Amount	$ 761,998	$ 532,621
Debt Instrument, Interest Rate During Period	0.25%	0.15%
Long-term Debt [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Short-term Debt, Weighted Average Interest Rate	1.05%	0.98%
Call Option [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 103,977
Long-term Debt, Maturities, Repayments of Principal in Year Three	801,930
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	179,803
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 219,333
Long-term Debt, Weighted Average Interest Rate	1.02%
Due after one year	$ 1,305,043
Due year two [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.02%	1.16%
Due year two [Member] | Call Option [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.77%
Due year three [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.85%	0.90%
Due year three [Member] | Call Option [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.43%
Due year four [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.73%	1.28%
Due year four [Member] | Call Option [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	0.00%
Due year five [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.68%	1.42%
Due year five [Member] | Call Option [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.90%
Due after five years [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.05%	3.09%
Due after five years [Member] | Call Option [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Weighted Average Interest Rate	2.59%