Revenue, Contract Assets and Liabilities - Contract Balances (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Contract Assets (Unearned Income)
Current contract assets	$ 197		$ 177
Long-term contract assets	14		7
Current unearned income	(103)		(112)
Long-term unearned income	(29)		(32)
Net Contract Assets (Unearned Income)	79		40
Accounts receivable, net	820		$ 782
Revenue recognized	$ 53	$ 83

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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