Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Level 1 [Member]
Assets
Total cash equivalents and marketable securities	$ 125,090	$ 20,354
Level 1 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities	125,090	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	176,645	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	49,466	46,624
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	99,179	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