Summary of Significant Accounting Policies (Restricted Cash) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 64,093	$ 123,575	$ 156,632
Restricted cash	3,000	3,000	3,000
Total cash, cash equivalents and restricted cash in the Condensed Statements of Cash Flows	$ 67,093	$ 126,575	$ 159,632	$ 77,627