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Convertible 3.75% Senior Notes (Details Textual) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 22, 2014	Dec. 16, 2014
Convertible Notes Payable
Debt Instrument [Line Items]
Stated interest rate	3.75%		3.75%
Remaining term of the debt	1 year 9 months
Qualified Institutional Buyers
Debt Instrument [Line Items]
Face amount of the Notes		$ 18,750,000.00	$ 125,000,000