Debt - Repayments of Principal and Interest (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt	$ 80,717	$ 64,563
Interest payments in 2018	2,373
Interest payments in 2019	1,653
Interest payments in 2020	1,149
Interest payments in 2021	787
Interest payments in 2022	532
Interest payments thereafter	1,144
Interest payments	7,638
Total repayments in 2018	26,875
Total repayments in 2019	20,142
Total repayments in 2020	15,845
Total repayments in 2021	8,589
Total repayments in 2022	5,855
Total repayments thereafter	11,621
Total repayments of principal and interest	88,927
Secured Debt [Member]
Debt Instrument [Line Items]
Repayments of principal in 2018	19,348
Repayments of principal in 2019	12,406
Repayments of principal in 2020	6,203
Repayments of principal in 2021	1,775
Repayments of principal in 2022	227
Repayments of principal thereafter	0
Long-term Debt	39,959
Unsecured Debt [Member]
Debt Instrument [Line Items]
Repayments of principal in 2018	5,154
Repayments of principal in 2019	6,083
Repayments of principal in 2020	8,493
Repayments of principal in 2021	6,027
Repayments of principal in 2022	5,096
Repayments of principal thereafter	10,477
Long-term Debt	$ 41,330

Amount of interest payments scheduled after the fifth fiscal year following the latest fiscal year.

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Amount of interest payments scheduled in the next fiscal year following the latest fiscal year.

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Amount of interest payments scheduled in the fifth fiscal year following the latest fiscal year.

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Amount of interest payments scheduled in the fourth fiscal year following the latest fiscal year.

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Amount of interest payments scheduled in the third fiscal year following the latest fiscal year.

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Amount of interest payments scheduled in the second fiscal year following the latest fiscal year.

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Amount of principal and interest payments scheduled after the fifth fiscal year following the latest fiscal year.

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Amount of principal and interest payments scheduled in the next fiscal year following the latest fiscal year.

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Amount of principal and interest payments scheduled in the fifth fiscal year following the latest fiscal year.

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Amount of principal and interest payments scheduled in the fourth fiscal year following the latest fiscal year.

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Amount of principal and interest payments scheduled in the third fiscal year following the latest fiscal year.

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Amount of principal and interest payments scheduled in the second fiscal year following the latest fiscal year.

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Repayments of Principal and Interest

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of interest payable on debt, including, but not limited to, trade payables.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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-Publisher FASB
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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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