Deferred Revenue - Summary of Changes in Deferred Revenue (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Deferred Revenue Disclosure [Abstract]
Balance at beginning of period	$ 834	$ 1,215
Deferral of revenue	544	559
Recognition of deferred revenue	(430)	(847)
Balance at end of period	$ 948	$ 927