Revenue - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Contract assets	$ 0		$ 0		$ 0
Deferred revenue	20,783,000		20,783,000		17,426,000
Interest income earned	4,300	$ 3,000	8,600	$ 8,000
Courses in Progress
Disaggregation of Revenue [Line Items]
Future revenue	12,000,000.0		12,000,000.0		9,600,000
Future Courses
Disaggregation of Revenue [Line Items]
Future revenue	$ 8,800,000		$ 8,800,000		$ 7,800,000