Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Level 1 [Member]
Assets
Total cash equivalents and marketable securities	$ 71,205	$ 20,354
Level 1 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities	71,205	20,354
Level 1 [Member] | Government-backed security [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 1 [Member] | Asset-backed securities [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 1 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 2 [Member]
Assets
Total cash equivalents and marketable securities	213,299	143,221
Level 2 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 2 [Member] | Government-backed security [Member]
Assets
Total cash equivalents and marketable securities	28,000	7,500
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total cash equivalents and marketable securities	17,501	46,624
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	167,798	89,097
Level 3 [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 3 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 3 [Member] | Government-backed security [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 3 [Member] | Asset-backed securities [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 3 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	$ 0	$ 0