SEGMENT INFORMATION - Operating Results by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Revenue	$ 47,179	$ 44,611
Cost of revenue	11,101	8,426
Sales and marketing	25,434	23,238
Research and development	6,917	5,738
General and administrative	9,562	8,692
Loss from operations	(5,835)	(1,483)
Unallocated non-operating income	(34)	(769)
Loss before income taxes	(5,801)	(714)
Operating Segments
Segment Reporting Information [Line Items]
Cost of revenue	7,643	5,910
Sales and marketing	24,438	22,618
Research and development	6,324	5,646
General and administrative	734	449
Loss from operations	8,040	9,988
Segment Reconciling Items
Segment Reporting Information [Line Items]
Cost of revenue	3,458	2,516
Sales and marketing	996	620
Research and development	593	92
General and administrative	1,963	1,637
Subtotal	7,010	4,865
Corporate, Non-Segment
Segment Reporting Information [Line Items]
General and administrative	6,865	6,606
Subtotal	6,831	5,837
Unallocated non-operating income	(34)	(769)
Literacy
Segment Reporting Information [Line Items]
Revenue	$ 17,486	$ 14,806
Segment contribution margin %	16.10%	20.30%
Literacy \| Operating Segments
Segment Reporting Information [Line Items]
Cost of revenue	$ 2,936	$ 2,458
Sales and marketing	8,039	6,909
Research and development	3,142	1,995
General and administrative	552	431
Loss from operations	2,817	3,013
E&E Language
Segment Reporting Information [Line Items]
Revenue	$ 13,552	$ 14,443
Segment contribution margin %	40.60%	42.30%
E&E Language \| Operating Segments
Segment Reporting Information [Line Items]
Cost of revenue	$ 2,271	$ 1,469
Sales and marketing	5,807	6,846
General and administrative	(24)	21
Loss from operations	5,498	6,107
Consumer Language
Segment Reporting Information [Line Items]
Revenue	$ 16,141	$ 15,362
Segment contribution margin %	18.00%	29.60%
Consumer Language \| Operating Segments
Segment Reporting Information [Line Items]
Cost of revenue	$ 2,437	$ 1,984
Sales and marketing	10,588	8,833
General and administrative	206	(3)
Loss from operations	2,910	4,548
Shared Services \| Operating Segments
Segment Reporting Information [Line Items]
Cost of revenue	(1)	(1)
Sales and marketing	4	30
Research and development	3,182	3,651
Loss from operations	(3,185)	(3,680)
Combined Language
Segment Reporting Information [Line Items]
Revenue	$ 29,693	$ 29,805
Segment contribution margin %	17.60%	23.40%
Combined Language \| Operating Segments
Segment Reporting Information [Line Items]
Cost of revenue	$ 4,707	$ 3,452
Sales and marketing	16,399	15,709
Research and development	3,182	3,651
General and administrative	182	18
Loss from operations	$ 5,223	$ 6,975

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