Summary of Significant Accounting Policies (Cash and Equivalents) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 106,307	$ 144,353	$ 93,108	$ 32,335
Cash [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	10,482	17,481
Money market funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	36,762	95,896
Asset-backed Securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	16,007	19,000
Corporate debt securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 43,056	$ 11,976