Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Level 1 [Member]
Assets
Total cash equivalents and marketable securities	$ 20,354	$ 101,839
Level 1 [Member] | Auction Rate Securities [Member]
Assets
Total cash equivalents and marketable securities	0	1,790
Level 1 [Member] | Money market funds [Member]
Assets
Total cash equivalents and marketable securities	20,354	100,049
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	143,221	26,978
Level 2 [Member] | Auction Rate Securities [Member]
Assets
Total cash equivalents and marketable securities	0	0
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	7,500	0
Level 2 [Member] | Asset-backed securities [Member]
Assets
Total cash equivalents and marketable securities	46,624	0
Level 2 [Member] | Corporate Debt Securities [Member]
Assets
Total cash equivalents and marketable securities	89,097	26,978
Level 3 [Member]
Assets
Total cash equivalents and marketable securities	0	0
Level 3 [Member] | Auction Rate Securities [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