Investment in Securities, Securities Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Trading securities [Abstract]
Investment sold, not yet purchased	$ 0	$ 0

Value of the short position.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-12A.2,Column C)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611177-123010

Name:	us-gaap_InvestmentSoldNotYetPurchasedAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant