Summary of Significant Accounting Policies (Cash and Equivalents) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 95,847	$ 144,353	$ 132,225	$ 93,108
Cash [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	15,540	17,481
Money market funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	51,816	95,896
Government-backed securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	19,000	19,000
Asset-backed Securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	3,500	0
Corporate debt securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 5,991	$ 11,976