Deferred Revenue - Summary of Deferred Revenue Activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Movement in Deferred Revenue [Roll Forward]
Balance at beginning of period	$ 1,212
Additions to deferred revenue	1,200
Recognized as revenue	(1,655)
Balance at end of period	757
BED
Movement in Deferred Revenue [Roll Forward]
Balance at beginning of period	644
Additions to deferred revenue	0
Recognized as revenue	(80)
Balance at end of period	564
Grants
Movement in Deferred Revenue [Roll Forward]
Balance at beginning of period	568
Additions to deferred revenue	1,200
Recognized as revenue	(1,575)
Balance at end of period	$ 193