Notes Payable - Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Due within one year	$ 8,089,826	$ 8,440,123
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.24%	0.70%
Long-term Debt, Current Maturities	$ 3,804,646	$ 2,155,784
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 2,644,734	$ 1,733,121
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.91%	1.68%
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 1,842,143	$ 1,283,869
Long-term Debt, Maturities, Repayments of Principal in Year Four	849,263	495,096
Long-term Debt, Maturities, Repayments of Principal in Year Five	790,564	602,037
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,306,086	1,108,854
Long-term Debt, Excluding Current Maturities	7,432,790	5,222,977
Debt and Capital Lease Obligations	$ 15,522,616	$ 13,663,100
Debt, Weighted Average Interest Rate	1.56%	1.07%
Debt Instrument, Redemption, Period Two [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.48%	1.08%
Debt Instrument, Redemption, Period Three [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.68%	1.32%
Debt Instrument, Redemption, Period Four [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.85%	1.50%
Debt Instrument, Redemption, Period Five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.03%	1.83%
Debt Instrument, Redemption, After Period Five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.05%	3.01%
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Due within one year	$ 1,724,969	$ 3,789,137
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.20%	0.59%
Short-term Debt, Average Outstanding Amount	$ 2,262,582	$ 5,753,425
Debt Instrument, Interest Rate During Period	0.86%	0.50%
Medium-term Notes [Member]
Debt Instrument [Line Items]
Due within one year	$ 2,560,211	$ 2,495,202
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.27%	0.70%
Short-term Debt, Average Outstanding Amount	$ 2,885,966	$ 1,551,094
Debt Instrument, Interest Rate During Period	1.04%	0.57%
Long-term Debt [Member]
Debt Instrument [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.23%	0.90%