Summary of Significant Accounting Principles (Details-1) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents	$ 95,579	$ 107,952
Restricted cash	3,516	1,635
Receivable - margin account	1,172	2,507
Cash, cash equivalents, and restricted cash	100,267	112,094	$ 107,970	$ 101,633
MGE [Member]
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents	3,128	5,951
Restricted cash	3,516	1,635
Receivable - margin account	1,172	2,507
Cash, cash equivalents, and restricted cash	$ 7,816	$ 10,093	$ 10,885	$ 16,442