Investment in Securities, Securities Sold, Not Yet Purchased (Details) - Investment in Securities [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Debt and equity securities [Abstract]
Proceeds	$ 4,726	$ 4,863	$ 7,280
Fair value	5,211	5,731	7,519
Debt and Equity Securities [Member] | Common Stocks [Member]
Debt and equity securities [Abstract]
Proceeds	4,726	4,862	7,279
Fair value	4,844	5,396	7,467
Debt and Equity Securities [Member] | Other Investments [Member]
Debt and equity securities [Abstract]
Proceeds	0	1	1
Fair value	$ 367	$ 335	$ 52