Summary of Significant Accounting Policies (Details Textual) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Restricted cash	$ 2,800		$ 4,300
Proceeds from sale/maturity of short-term investments	$ 150,768	$ 107,816
Maximum
Summary Of Significant Accounting Policies [Line Items]
Short-term investment contractual maturities	2 years		2 years