Revenue Recognition - Reconciliation of Contract Balances (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Reconciliation of Contract Balances
Notes and accounts receivable - trade (net of allowances)	$ 7,071	$ 7,071	$ 8,295	$ 8,928
Notes and accounts receivable - trade, allowances	310	310	297	297
Contract assets	510	510	557
Deferred income (current)	10,704	10,704	11,493	11,552
Deferred income (noncurrent)	3,507	3,507	$ 3,758	$ 3,746
Revenue recognized that was included in deferred income at the beginning of the period	$ 3,800	$ 8,000

Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

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A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as noncurrent.

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Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

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