Summary of Significant Accounting Principles (Details-1) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents	$ 133,855	$ 107,952
Restricted cash	406	1,635
Receivable - margin account	780	2,507
Cash, cash equivalents, and restricted cash	135,041	112,094	$ 108,062	$ 101,633
MGE [Member]
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents	47,264	5,951
Restricted cash	406	1,635
Receivable - margin account	780	2,507
Cash, cash equivalents, and restricted cash	$ 48,450	$ 10,093	$ 5,283	$ 16,442