Stock Option Plans, Allocation of Recognized Period Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
All stock-based compensation expense [Abstract]
Total stock-based compensation expense	$ 905	$ 2,220	$ 1,930	$ 5,593
Research and Development [Member]
All stock-based compensation expense [Abstract]
Total stock-based compensation expense	166	579	496	1,785
General and Administrative [Member]
All stock-based compensation expense [Abstract]
Total stock-based compensation expense	$ 739	$ 1,641	$ 1,434	$ 3,808