Basis of Presentation and Significant Accounting Policies - Revenue from Data Acquisition (Details)	12 Months Ended
Dec. 31, 2017
Minimum [Member]
Revenue from External Customer
Average Duration of the Data Creation Process	12 months
Maximum [Member]
Revenue from External Customer
Average Duration of the Data Creation Process	18 months

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration