Summary of Significant Accounting Policies - Change in Unbilled Contract and Unearned Revenues (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Unbilled contract revenue
Beginning balance	$ 58
Additions	111
Subtractions	138
Ending balance	31	$ 58
Unearned revenue
Beginning balance	87
Additions	34
Subtractions	105
Ending balance	$ 16	87
Contracts with customer, revenue recognized, amount outstanding end of last period		$ 50

Contract With Customer Liability, Revenue Recognized, Previously Outstanding

No definition available.

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Balance Type:	credit
Period Type:	duration

- Definition

Contract with Customer, Liability, Decrease From Billings

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Unbilled Contracts Receivable, Decrease From Billings

+ References

No definition available.

+ Details

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- Definition

Unbilled Contracts Receivable, Revenue Recognized

+ References

No definition available.

+ Details

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- References

No definition available.

+ Details

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- References

No definition available.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

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-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

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- Definition

Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 912
-SubTopic 310
-Section 45
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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