Segment Information - Schedule of segment reporting information (Details) $ in Millions	3 Months Ended			6 Months Ended
	Nov. 30, 2020 USD ($)	Aug. 31, 2020 USD ($)	Nov. 30, 2019 USD ($)	Nov. 30, 2020 USD ($) segment	Nov. 30, 2019 USD ($)	May 31, 2020 USD ($)	May 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment				3
Revenues	$ 406.2		$ 597.2	$ 621.4	$ 829.8
Bad debt expense	2.1	$ 1.4	2.7	3.5	4.3
Depreciation and amortization	23.4		22.6	46.1	45.1
Segment operating income (loss)	48.8		105.1	(8.2)	17.7
Segment assets	2,106.9		2,031.7	2,106.9	2,031.7
Goodwill	125.6		125.4	125.6	125.4	$ 124.9	$ 125.2
Expenditures for other non-current assets	24.0		36.1	53.5	69.2
Other non-current assets	868.3		954.9	$ 868.3	954.9
Children's Book Publishing and Distribution
Segment Reporting Information [Line Items]
Number of operating segments | segment				3
Revenues	240.3		413.6	$ 331.2	523.2
Bad debt expense	(0.1)		1.1	0.1	1.6
Segment assets	607.1		674.6	607.1	674.6
Goodwill	47.3		47.2	47.3	47.2
Expenditures for other non-current assets	9.6		14.7	21.7	28.7
Other non-current assets	171.0		219.6	$ 171.0	219.6
Education
Segment Reporting Information [Line Items]
Number of operating segments | segment				3
Revenues	67.5		69.9	$ 121.1	118.3
Bad debt expense	1.3		0.8	1.4	0.8
Segment assets	212.8		196.1	212.8	196.1
Goodwill	68.2		68.2	68.2	68.2
Expenditures for other non-current assets	3.2		5.0	6.3	9.6
Other non-current assets	123.7		122.4	123.7	122.4
Overhead
Segment Reporting Information [Line Items]
Revenues	0.0		0.0	0.0	0.0
Bad debt expense	0.0		0.0	0.0	0.0
Segment assets	957.4		846.8	957.4	846.8
Goodwill	0.0		0.0	0.0	0.0
Expenditures for other non-current assets	8.1		10.6	18.9	18.2
Other non-current assets	487.2		514.3	487.2	514.3
Total Domestic
Segment Reporting Information [Line Items]
Revenues	307.8		483.5	452.3	641.5
Bad debt expense	1.2		1.9	1.5	2.4
Segment assets	1,777.3		1,717.5	1,777.3	1,717.5
Goodwill	115.5		115.4	115.5	115.4
Expenditures for other non-current assets	20.9		30.3	46.9	56.5
Other non-current assets	781.9		856.3	$ 781.9	856.3
International
Segment Reporting Information [Line Items]
Number of operating segments | segment				3
Revenues	98.4		113.7	$ 169.1	188.3
Bad debt expense	0.9		0.8	2.0	1.9
Goodwill	10.1		10.0	10.1	10.0
Expenditures for other non-current assets	3.1		5.8	6.6	12.7
Expenditures to acquire long-lived assets	1.5		4.2	3.5	9.9
Long-lived assets	46.6		66.8	46.6	66.8
Operating Segments | Children's Book Publishing and Distribution
Segment Reporting Information [Line Items]
Depreciation and amortization	6.6		6.5	13.2	13.2
Segment operating income (loss)	37.7		109.6	8.5	67.9
Operating Segments | Education
Segment Reporting Information [Line Items]
Depreciation and amortization	3.3		3.4	6.4	6.5
Segment operating income (loss)	11.9		6.2	9.7	(7.2)
Operating Segments | Overhead
Segment Reporting Information [Line Items]
Depreciation and amortization	11.9		10.9	23.4	21.9
Segment operating income (loss)	(20.0)		(22.4)	(50.8)	(51.0)
Operating Segments | Total Domestic
Segment Reporting Information [Line Items]
Depreciation and amortization	21.8		20.8	43.0	41.6
Segment operating income (loss)	29.6		93.4	(32.6)	9.7
Operating Segments | International
Segment Reporting Information [Line Items]
Depreciation and amortization	1.6		1.8	3.1	3.5
Segment operating income (loss)	19.2		11.7	24.4	8.0
Segment assets	329.6		314.2	329.6	314.2
Other non-current assets	$ 86.4		$ 98.6	$ 86.4	$ 98.6