Summary of Significant Accounting Policies (Cash and Equivalents) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 113,402	$ 106,307
Cash [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	13,448	10,482
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	31,056	36,762
Asset-backed Securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	18,750	16,007
Corporate debt securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 50,148	$ 43,056