Accounting Policies - Unearned Revenue (Details) - USD ($) $ in Billions	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Unearned revenue		$ 6.1
Unearned revenue, revenue recognized from beginning balance	$ 3.9
Unearned revenue, noncurrent	1.2	$ 1.0
Remaining performance obligation, contracts exceeding one year	$ 16.0
Remaining performance obligation, expected timing of satisfaction, weighted average remaining life	3 years 6 months

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Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

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The noncurrent portion of deferred revenue amount as of balance sheet date. Deferred revenue is a liability related to a revenue producing activity for which revenue has not yet been recognized, and is not expected to be recognized in the next twelve months. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount of revenue recognized that was previously reported as deferred or unearned revenue.

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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.

+ References

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