Segment Information - Schedule of segment reporting information (Details) $ in Millions	3 Months Ended
	Aug. 31, 2020 USD ($) segment	Aug. 31, 2019 USD ($)	May 31, 2020 USD ($)	May 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	3
Revenues	$ 215.2	$ 232.6
Bad debt expense	1.4	1.6
Depreciation and amortization	22.7	22.5
Segment operating income (loss)	(57.0)	(87.4)
Segment assets	2,056.3	1,944.6
Goodwill	125.6	125.0	$ 124.9	$ 125.2
Expenditures for other non-current assets	29.5	33.1
Other non-current assets	$ 869.1	952.2
Children's Book Publishing and Distribution
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Revenues	$ 90.9	109.6
Bad debt expense	0.2	0.5
Segment assets	555.4	651.9
Goodwill	47.4	46.8
Expenditures for other non-current assets	12.1	14.0
Other non-current assets	$ 172.6	219.2
Education
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Revenues	$ 53.6	48.4
Bad debt expense	0.1	0.0
Segment assets	215.6	198.7
Goodwill	68.2	68.2
Expenditures for other non-current assets	3.1	4.6
Other non-current assets	123.9	120.0
Overhead
Segment Reporting Information [Line Items]
Revenues	0.0	0.0
Bad debt expense	0.0	0.0
Segment assets	985.8	808.4
Goodwill	0.0	0.0
Expenditures for other non-current assets	10.8	7.6
Other non-current assets	491.1	520.0
Total Domestic
Segment Reporting Information [Line Items]
Revenues	144.5	158.0
Bad debt expense	0.3	0.5
Expenditures for other non-current assets	$ 26.0	26.2
International
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Revenues	$ 70.7	74.6
Bad debt expense	1.1	1.1
Goodwill	10.0	10.0
Expenditures for other non-current assets	3.5	6.9
Expenditures to acquire long-lived assets	2.0	5.7
Long-lived assets	46.7	64.5
Operating Segments | Children's Book Publishing and Distribution
Segment Reporting Information [Line Items]
Depreciation and amortization	6.6	6.7
Segment operating income (loss)	(29.2)	(41.7)
Operating Segments | Education
Segment Reporting Information [Line Items]
Depreciation and amortization	3.1	3.1
Segment operating income (loss)	(2.2)	(13.4)
Operating Segments | Overhead
Segment Reporting Information [Line Items]
Depreciation and amortization	11.5	11.0
Segment operating income (loss)	(30.8)	(28.6)
Operating Segments | Total Domestic
Segment Reporting Information [Line Items]
Depreciation and amortization	21.2	20.8
Segment operating income (loss)	(62.2)	(83.7)
Segment assets	1,756.8	1,659.0
Goodwill	115.6	115.0
Other non-current assets	787.6	859.2
Operating Segments | International
Segment Reporting Information [Line Items]
Depreciation and amortization	1.5	1.7
Segment operating income (loss)	5.2	(3.7)
Segment assets	299.5	285.6
Other non-current assets	$ 81.5	$ 93.0