GOODWILL - Schedule of Goodwill Balance and Changes (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Goodwill [Line Items]
Gross Goodwill, beginning balance	$ 76,472
Accumulated Impairment, beginning balance	(27,514)
Goodwill, beginning balance	48,958
Effect of change in foreign currency rate	(151)
Gross Goodwill, ending balance	76,321
Accumulated Impairment, ending balance	(27,514)
Goodwill, ending balance	48,807
Literacy
Goodwill [Line Items]
Gross Goodwill, beginning balance	9,962
Goodwill, beginning balance	9,962
Gross Goodwill, ending balance	9,962
Goodwill, ending balance	9,962
E&E Language
Goodwill [Line Items]
Gross Goodwill, beginning balance	38,996
Goodwill, beginning balance	38,996
Effect of change in foreign currency rate	(151)
Gross Goodwill, ending balance	38,845
Goodwill, ending balance	38,845
Consumer Language
Goodwill [Line Items]
Gross Goodwill, beginning balance	27,514
Accumulated Impairment, beginning balance	(27,514)
Gross Goodwill, ending balance	27,514
Accumulated Impairment, ending balance	$ (27,514)