Fair Value Measurements (Fair value hierarchy for its financial assets and liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Level 1 [Member]
Assets
Total cash equivalents and marketable securities	$ 156,158	$ 20,354
Level 1 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities	156,158	20,354
Level 1 [Member] | U.S. agency debt securities [Member]
Assets
Total cash equivalents and marketable securities	0	0
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	166,406	143,221
Level 2 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 2 [Member] | U.S. agency debt securities [Member]
Assets
Total cash equivalents and marketable securities	35,721	0
Level 2 [Member] | Government-backed security [Member]
Assets
Total cash equivalents and marketable securities	18,000	7,500
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total cash equivalents and marketable securities	46,639	46,624
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	66,046	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] | U.S. agency debt securities [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