Stock-Based Compensation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Costs and expenses:
Total stock-based compensation expense	$ 1,378	$ 1,224	$ 2,707	$ 2,607
Research and development [Member]
Costs and expenses:
Total stock-based compensation expense	723	646	1,408	1,421
General and administrative [Member]
Costs and expenses:
Total stock-based compensation expense	$ 655	$ 578	$ 1,299	$ 1,186