Notes Payable - Earliest Interest Reset Date of Borrowing Outstanding (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Schedule of Short-term and Long-term Borrowings [Line Items]
Meduim Term Notes Called During the Period	$ 2,400,000	$ 500,000
Due within one year	$ 9,111,461	$ 7,353,953
Short-term Debt, Weighted Average Interest Rate	0.48%	0.32%
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
Debt and Capital Lease Obligations	$ 14,078,497	$ 12,825,139
Long-term Debt, Weighted Average Interest Rate	1.59%	1.56%
Debt, Weighted Average Interest Rate	0.87%	0.85%
Due within one year [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Due within one year	$ 10,560,504
Short-term Debt, Weighted Average Interest Rate	0.55%
Due year two [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 1,071,581
Long-term Debt, Weighted Average Interest Rate	1.06%
Due year three [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 737,333
Long-term Debt, Weighted Average Interest Rate	1.32%
Due year four [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Four	$ 520,284
Long-term Debt, Weighted Average Interest Rate	1.73%
Due year five [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Five	$ 298,764
Long-term Debt, Weighted Average Interest Rate	1.76%
Due after five years [Member]
Schedule of Short-term and Long-term Borrowings [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 890,031
Long-term Debt, Weighted Average Interest Rate	3.34%