Segment Information - Schedule of segment reporting information (Details) $ in Millions		3 Months Ended				9 Months Ended
		Feb. 28, 2018 USD ($)		Feb. 28, 2017 USD ($)		Feb. 28, 2018 USD ($) segment	Feb. 28, 2017 USD ($)	May 31, 2017 USD ($)	May 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment						3
Revenues		$ 344.7		$ 336.2		$ 1,132.2	$ 1,242.0
Bad debt expense		1.7		1.6		7.9	9.3
Depreciation and amortization	[1]	17.0		15.0		48.3	45.8
Asset impairments		4.3		0.0		11.0	0.0
Segment operating income (loss)		(23.7)		(24.7)		(18.3)	24.9
Segment assets		1,833.7		1,847.2		1,833.7	1,847.2
Goodwill		119.1		116.2		119.1	116.2	$ 118.9	$ 116.2
Expenditures for other non-current assets		57.6	[2]	34.1	[2]	146.5	97.0
Other non-current assets	[2]	793.7		695.6		$ 793.7	695.6
Children's Book Publishing and Distribution
Segment Reporting Information [Line Items]
Number of operating segments | segment						3
Revenues						$ 678.0	769.3
Bad debt expense						3.4	3.3
Segment assets		494.8		478.4		494.8	478.4
Goodwill		40.9		40.9		40.9	40.9
Expenditures for other non-current assets						44.9	52.2
Other non-current assets	[2]	152.0		146.9		$ 152.0	146.9
Education
Segment Reporting Information [Line Items]
Number of operating segments | segment						2
Revenues						$ 177.6	186.4
Bad debt expense						1.4	0.9
Segment assets		179.1		158.8		179.1	158.8
Goodwill		68.2		65.4		68.2	65.4
Expenditures for other non-current assets						12.5	7.8
Other non-current assets	[2]	99.7		83.7		99.7	83.7
Overhead
Segment Reporting Information [Line Items]
Revenues	[3]					0.0	0.0
Bad debt expense	[3]						0.0
Segment assets	[3]	886.1		948.7		886.1	948.7
Goodwill	[3]	0.0		0.0		0.0	0.0
Expenditures for other non-current assets	[3]					78.4	28.5
Other non-current assets	[2],[3]	466.5		398.6		466.5	398.6
Total Domestic
Segment Reporting Information [Line Items]
Revenues						855.6	955.7
Bad debt expense						4.8	4.2
Segment assets		1,560.0		1,585.9		1,560.0	1,585.9
Goodwill		109.1		106.3		109.1	106.3
Expenditures for other non-current assets						135.8	88.5
Other non-current assets	[2]	718.2		629.2		$ 718.2	629.2
International
Segment Reporting Information [Line Items]
Number of operating segments | segment						3
Revenues						$ 276.6	286.3
Bad debt expense						3.1	5.1
Segment assets		273.7		261.3		273.7	261.3
Goodwill		10.0		9.9		10.0	9.9
Expenditures for other non-current assets						10.7	8.5
Other non-current assets	[2]	75.5		66.4		75.5	66.4
Operating Segments | Children's Book Publishing and Distribution
Segment Reporting Information [Line Items]
Revenues		199.4		199.0
Bad debt expense		0.7		0.1
Depreciation and amortization	[1]	5.4		5.3		15.8	16.6
Asset impairments		0.0				0.0
Segment operating income (loss)		(0.9)		6.3		55.3	91.2
Expenditures for other non-current assets	[2]	17.2		14.9
Operating Segments | Education
Segment Reporting Information [Line Items]
Revenues		61.7		60.1
Bad debt expense		0.4		0.4
Depreciation and amortization	[1]	2.3		2.1		6.6	6.2
Asset impairments		0.0				0.0
Segment operating income (loss)		(0.2)		3.5		(8.9)	7.8
Expenditures for other non-current assets	[2]	5.0		2.8
Operating Segments | Overhead
Segment Reporting Information [Line Items]
Revenues	[3]	0.0		0.0
Bad debt expense	[3]	0.0		0.0
Depreciation and amortization	[1],[3]	7.5		6.0		20.7	17.4
Asset impairments	[3]	4.3				11.0
Segment operating income (loss)	[3]	(23.3)		(30.8)		(77.3)	(91.3)
Expenditures for other non-current assets	[2],[3]	29.7		13.9
Operating Segments | Total Domestic
Segment Reporting Information [Line Items]
Revenues		261.1		259.1
Bad debt expense		1.1		0.5
Depreciation and amortization	[1]	15.2		13.4		43.1	40.2
Asset impairments		4.3				11.0
Segment operating income (loss)		(24.4)		(21.0)		(30.9)	7.7
Expenditures for other non-current assets	[2]	51.9		31.6
Operating Segments | International
Segment Reporting Information [Line Items]
Revenues		83.6		77.1
Bad debt expense		0.6		1.1
Depreciation and amortization	[1]	1.8		1.6		5.2	5.6
Asset impairments		0.0				0.0
Segment operating income (loss)		0.7		(3.7)		12.6	17.2
Expenditures for other non-current assets	[2]	5.7		2.5
Expenditures to acquire long-lived assets		3.6		1.5		6.6	4.8
Long-lived assets		35.8		33.4		35.8	33.4
United States | Pension Plans
Segment Reporting Information [Line Items]
Benefit cost of settlement event		$ (39.6)		$ 0.0		$ (55.0)	$ 0.0

[1] Includes depreciation of property, plant and equipment and amortization of intangible assets and prepublication and production costs.

[2] Other non-current assets include property, plant and equipment, prepublication assets, production assets, royalty advances, goodwill, intangible assets and investments. Expenditures for other non-current assets for the International reportable segment include expenditures for long-lived assets of $3.6 and $1.5 for the three months and $6.6 and $4.8 for the nine months ended February 28, 2018 and February 28, 2017, respectively. Other non-current assets for the International reportable segment include long-lived assets of $35.8 and $33.4 as of February 28, 2018 and February 28, 2017, respectively.

[3] Overhead includes all domestic corporate amounts not allocated to segments, including expenses and costs related to the management of corporate assets. Unallocated assets are principally comprised of deferred income taxes and property, plant and equipment related to the Company’s headquarters in the metropolitan New York area, its fulfillment and distribution facilities located in Missouri and its facility located in Connecticut.