Leases - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Leases
Net book value of equipment under capital lease	$ 850	$ 775
Total operating lease expense	$ 1,716	$ 1,718	$ 1,627

The total gross amount less the charge for the use of the long-lived depreciable assets subject to a lease meeting the criteria for capitalization.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84164817&loc=d3e45280-112737

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