Notes Payable - Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 4,344,285	$ 3,804,646
Due within one year	7,757,050	8,089,826
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 3,090,405	$ 2,644,734
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.48%	1.91%
Long-term Debt, Excluding Current Maturities	$ 8,486,647	$ 7,432,790
Due after one year	8,486,647	7,432,790
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,434,383	1,306,086
Long-term Debt, Maturities, Repayments of Principal in Year Five	881,738	790,564
Long-term Debt, Maturities, Repayments of Principal in Year Four	859,470	849,263
Long-term Debt, Maturities, Repayments of Principal in Year Three	2,220,651	1,842,143
Debt and Capital Lease Obligations	$ 16,243,697	$ 15,522,616
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.10%	1.24%
Debt, Weighted Average Interest Rate	2.30%	1.56%
Debt Instrument, Redemption, Period Two [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.11%	1.48%
Debt Instrument, Redemption, Period Three [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.41%	1.68%
Debt Instrument, Redemption, Period Four [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.19%	1.85%
Debt Instrument, Redemption, Period Five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.88%	2.03%
Debt Instrument, Redemption, After Period Five [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.34%	3.05%
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Due within one year	$ 1,586,385	$ 1,724,969
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.35%	1.20%
Short-term Debt, Average Outstanding Amount	$ 1,432,470	$ 2,262,582
Debt Instrument, Interest Rate During Period	1.83%	0.86%
Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 239,327
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.66%
Due after one year	$ 1,363,244
Long-term Debt, Maturities, Repayments of Principal after Year Five	294,720
Long-term Debt, Maturities, Repayments of Principal in Year Five	253,758
Long-term Debt, Maturities, Repayments of Principal in Year Four	165,810
Long-term Debt, Maturities, Repayments of Principal in Year Three	409,629
Medium-term Notes [Member]
Debt Instrument [Line Items]
Due within one year	$ 1,826,380	$ 2,560,211
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.29%	1.27%
Short-term Debt, Average Outstanding Amount	$ 1,977,445	$ 2,885,966
Debt Instrument, Interest Rate During Period	1.83%	1.04%
Long-term Debt [Member]
Debt Instrument [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.93%	1.23%
Due year two [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.13%
Due year three [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.38%
Due year four [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.31%
Due year five [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.14%
Due after five years [Member] | Call Option [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.27%