Inventories - Components of Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Components of Inventories
Raw materials	$ 20,361	$ 17,550
Inventories	$ 20,361	$ 17,550	[1]

[1] Contract costs primarily consist of deferred core services costs (including labor and vendor purchases) required to service future exhibitions, conferences and other events, and commission expenses incurred to obtain contracts. All such costs were previously included in “Inventories” and in certain other assets. As a result of the changes noted above, deferred core services costs related to exhibitions and events that are scheduled to occur longer than one year in the future are currently included in “Other investments and assets”. The impact of this change reduced total current assets at December 31, 2017 by $0.7 million. The amount of deferred core services costs included in “Other investments and assets” at June 30, 2018 was $5.1 million.