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.

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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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Amount, before 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.

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

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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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Amount of revenue recognized from performance obligation satisfied or partially satisfied in previous reporting periods. Includes, but is not limited to, change in transaction price.

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Summary of Significant Accounting Policies - Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Contract with Customer, Asset and Liability [Abstract]
Allowance for doubtful accounts for receivables
Contract assets
Receivables, net	1,019		885
Contract liabilities	57		$ 54
Revenue recognized that was included in the contract liability balance at the beginning of the reporting period
Revenue recognized from performance obligations satisfied in prior periods