Summary of Significant Accounting Principles (Details-1) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents	$ 101,157	$ 107,952
Restricted cash	2,763	1,635
Receivable - margin account	2,325	2,507
Cash, cash equivalents, and restricted cash	106,245	112,094	$ 122,728	$ 101,633
MGE [Member]
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents	4,152	5,951
Restricted cash	2,763	1,635
Receivable - margin account	2,325	2,507
Cash, cash equivalents, and restricted cash	$ 9,240	$ 10,093	$ 28,124	$ 16,442