Deferred Revenue - Summary of Changes in Deferred Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred Revenue Disclosure [Abstract]
Balance at beginning of period	$ 1,215	$ 1,411
Deferral of revenue	587	1,476
Recognition of deferred revenue	(968)	(1,672)
Balance at end of period	$ 834	$ 1,215