Investment in Securities, Securities Sold, Not Yet Purchased (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Investment in Securities [Abstract]
Investment sold, not yet purchased	$ 0	$ 0	$ 0
US Government Obligations [Member] | Convertible Notes [Member]
Investment [Line Items]
Restricted investments in securities held in escrow account	$ 0