Segment Information - Schedule of segment reporting information (Details) $ in Millions		3 Months Ended
		Aug. 31, 2018 USD ($) segment	Aug. 31, 2017 USD ($)	May 31, 2018 USD ($)	May 31, 2017 USD ($)	May 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment		3
Revenues		$ 218.4	$ 189.2
Bad debt expense		1.4	1.9
Depreciation and amortization	[1]	19.2	15.5
Asset impairments		0.0	6.7
Segment operating income (loss)		(83.8)	(101.8)
Segment assets		1,899.6	1,759.6
Goodwill		119.1	119.2	$ 119.2	$ 118.9	$ 118.9
Expenditures for other non-current assets		45.7	45.3
Other non-current assets	[2]	$ 834.3	732.4
Children's Book Publishing and Distribution
Segment Reporting Information [Line Items]
Number of operating segments | segment		3
Revenues		$ 93.3	66.8
Bad debt expense		0.8	0.5
Asset impairments	[3]		0.0
Segment assets		545.2	476.9
Goodwill		40.9	40.9
Expenditures for other non-current assets		10.4	9.9
Other non-current assets	[2]	$ 148.5	140.2
Education
Segment Reporting Information [Line Items]
Number of operating segments | segment		2
Revenues		$ 50.3	45.0
Bad debt expense		0.0	(0.1)
Asset impairments	[3]		0.0
Segment assets		192.3	176.1
Goodwill		68.2	68.3
Expenditures for other non-current assets		5.5	2.5
Other non-current assets	[2]	109.9	94.5
Overhead
Segment Reporting Information [Line Items]
Revenues	[3]	0.0	0.0
Bad debt expense	[3]	0.0	0.0
Asset impairments	[1],[3]		6.7
Segment assets	[3]	884.2	845.6
Goodwill	[3]	0.0	0.0
Expenditures for other non-current assets	[3]	25.5	30.8
Other non-current assets	[2],[3]	499.7	429.7
Total Domestic
Segment Reporting Information [Line Items]
Revenues		143.6	111.8
Bad debt expense		0.8	0.4
Asset impairments			6.7
Segment assets		1,621.7	1,498.6
Goodwill		109.1	109.2
Expenditures for other non-current assets		41.4	43.2
Other non-current assets	[2]	$ 758.1	664.4
International
Segment Reporting Information [Line Items]
Number of operating segments | segment		3
Revenues		$ 74.8	77.4
Bad debt expense		0.6	1.5
Asset impairments	[3]		0.0
Segment assets		277.9	261.0
Goodwill		10.0	10.0
Expenditures for other non-current assets		4.3	2.1
Other non-current assets	[2]	76.2	68.0
Operating Segments | Children's Book Publishing and Distribution
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]	5.3	5.2
Asset impairments		0.0
Segment operating income (loss)		(45.9)	(58.9)
Operating Segments | Education
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]	2.4	2.2
Asset impairments		0.0
Segment operating income (loss)		(15.0)	(12.5)
Operating Segments | Overhead
Segment Reporting Information [Line Items]
Depreciation and amortization	[1],[3]	9.9	6.4
Asset impairments	[3]	0.0
Segment operating income (loss)	[3]	(20.9)	(27.6)
Operating Segments | Total Domestic
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]	17.6	13.8
Asset impairments		0.0
Segment operating income (loss)		(81.8)	(99.0)
Operating Segments | International
Segment Reporting Information [Line Items]
Depreciation and amortization	[1]	1.6	1.7
Asset impairments		0.0
Segment operating income (loss)		(2.0)	(2.8)
Expenditures to acquire long-lived assets		2.8	1.3
Long-lived assets		$ 37.0	$ 33.9

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

[2] Other noncurrent assets include property, plant and equipment, prepublication assets, production assets, royalty advances, goodwill, intangible assets and investments. Expenditures for other noncurrent assets for the International reportable segment include expenditures for long-lived assets of $2.8 and $1.3 for the three months ended August 31, 2018 and August 31, 2017, respectively. Other noncurrent assets for the International reportable segment include long-lived assets of $37.0 and $33.9 as of August 31, 2018 and August 31, 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.