Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Significant Accounting Policies - Revenue Recognition [Abstract]
Unbilled revenue	$ 665	$ 643

Amount received for services rendered and products shipped, but not yet billed, for non-contractual agreements due within one year or the normal operating cycle, if longer.

No definition available.

Name:	us-gaap_UnbilledReceivablesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant