Fair Value Measurements - Assets and Liabilities Recurring and Non-recurring (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Recurring basis
Fair Value Measurements
Assets	$ 0	$ 0
Liabilities	0	0
Non-recurring basis
Fair Value Measurements
Assets	0	0
Liabilities	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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

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