Accounting Policies - Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Values of Financial Instruments
Foreign exchange hedges assets	$ 67	$ 111
Recurring \| Fair value \| Cash flows approach
Fair Values of Financial Instruments
Foreign exchange hedges assets	67	111
Recurring \| Fair value \| Significant Other Observable Inputs (Level 2) \| Cash flows approach
Fair Values of Financial Instruments
Foreign exchange hedges assets	$ 67	$ 111

Fair value as of the balance sheet date of all foreign currency derivative assets designated as cash flow hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

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