Notes Payable - Earliest Interest Reset Date of Borrowing Outstanding (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
Long-term Debt, Maturities, Repayments of Principal in Year Two	2,644,734	1,733,121
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
Debt and Capital Lease Obligations	$ 15,522,616	$ 13,663,100
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.24%	0.70%
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.91%	1.68%
Debt, Weighted Average Interest Rate	1.56%	1.07%
Meduim Term Notes Called During the Period	$ 0	$ 1,300,000
Due within one year [Member]
Debt Instrument [Line Items]
Due within one year	$ 9,291,949
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.28%
Due year two [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two	$ 2,176,327
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.49%
Due year three [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Three	$ 1,526,236
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.71%
Due year four [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Four	$ 744,336
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.89%
Due year five [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Five	$ 719,609
Long-term Debt, Weighted Average Interest Rate, at Point in Time	2.08%
Due after five years [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 1,064,159
Long-term Debt, Weighted Average Interest Rate, at Point in Time	3.31%

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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