Summary of Significant Accounting Policies (Cash and Equivalents) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 75,793	$ 144,353	$ 89,395	$ 93,108
Cash [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	17,266	17,481
Money market funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	41,527	95,896
Government-backed securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	17,000	19,000
Corporate debt securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 0	$ 11,976