Cash, Cash Equivalents and Available for Sale Securities (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Cash and cash equivalents	$ 145,991,000	$ 166,972,000	$ 140,917,000	$ 133,930,000
Cash	10,500,000	9,500,000
Investments in money market funds, U.S. Treasury securities and certain short-term investment-grade corporate debt securities with original maturities of 90 days or less	135,500,000	157,400,000
Available for sale securities with maturities of greater than 90 days	$ 0	$ 0