Financial Instruments and Fair Value Measurements - Expected Maturity (Details) - Restricted - Short-term bond	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Minimum
Financial instruments and fair value measurements
Investment term	0 years	0 years
Maximum
Financial instruments and fair value measurements
Investment term	3 years	3 years

Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

Name:	us-gaap_FairValueAssumptionsExpectedTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration