Revenue Recognition - Contract with Customer, Asset and Liability (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Customer receivables	$ 2,286	$ 2,346
Contract assets	134	140
Contract liabilities	869	860
Accounts receivable, gross	2,400	2,500
Allowance for doubtful accounts receivable	$ 145	$ 132

For unclassified balance sheet, amounts due from customers or clients for goods or services that have been delivered or sold in the normal course of business.

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- Definition

For an unclassified balance sheet, the amount due from customers or clients for goods or services that have been delivered or sold in the normal course of business, reduced to their estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

+ References

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For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

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- Definition

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.

+ References

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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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