Segment and Geographic Information Narrative (Details)	6 Months Ended
Jun. 30, 2018 site
U.S.
Segment Reporting Information [Line Items]
Distinct sites	5

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