Investment (Income), Net (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Feb. 28, 2019	Feb. 28, 2018	Feb. 28, 2019	Feb. 28, 2018
Other Income And Expenses [Abstract]
Interest (income)	$ (1,076)	$ (1,454)	$ (2,808)	$ (3,645)
Net (gain) loss on marketable securities	(1,683)	(1,935)	6,451	(6,833)
Dividend (income)	(1,967)	(2,082)	(3,769)	(3,185)
Investment (income), net	$ (4,726)	$ (5,471)	$ (126)	$ (13,663)

This item represents investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums, respectively, net of related investment expenses. This item also includes realized gains or losses on sales of investments in debt and equity securities, and other than temporary losses related to investments in debt and equity securities during the period. This account does not include investment income from real or personal property, such as rental income.

No definition available.

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Namespace Prefix:	rpm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration