Accounting Policies - Unearned Revenue (Details) - USD ($) $ in Billions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Unearned revenue		$ 6.1
Unearned revenue, revenue recognized from beginning balance	$ 4.9
Unearned Revenue, Noncurrent	1.2	$ 1.0
Remaining performance obligation, contracts exceeding one year	$ 17.8
Remaining performance obligation, expected timing of satisfaction, weighted average remaining life	3 years 6 months

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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 transaction price allocated to performance obligation that has not been recognized as revenue.

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Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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