Summary of Significant Accounting Policies (Cash and Equivalents) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 132,225	$ 93,108	$ 215,050	$ 32,335
Cash [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	7,182	29,569
Money market funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	49,321	14,950
U.S. Treasury securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	9,994	0
Government-backed security [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	39,500	20,000
Asset-backed Securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	0	8,185
Corporate debt securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 26,228	$ 20,404