Business Segment Information - Narrative (Details)	6 Months Ended
Jun. 30, 2018 segment vessel
Segment Reporting Information [Line Items]
Number of reportable segments \| segment	3
Robotics
Segment Reporting Information [Line Items]
Number of chartered vessels \| vessel	3

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

No definition available.

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration