Fair Value Measurements (Fair Value Hierarchy) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Assets
Money market funds	$ 100,049	$ 11,970
Total cash equivalents and investments	13,597	32,945
Level 1 [Member]
Assets
Auction rate security	1,790
Money market funds	100,049	11,970
Municipal bonds
Corporate debt securities
Total cash equivalents and investments	101,839	11,970
Liabilities:
Warrant liabilities
Level 2 [Member]
Assets
Auction rate security		1,584
Money market funds
Municipal bonds		1,671
Corporate debt securities	26,978	31,361
Total cash equivalents and investments	26,978	34,616
Liabilities:
Warrant liabilities
Level 3 [Member]
Assets
Auction rate security
Money market funds
Municipal bonds
Corporate debt securities
Total cash equivalents and investments
Liabilities:
Warrant liabilities		$ 267