Investments in Unconsolidated Affiliates - Financial Results of Four Largest Unconsolidated Affiliates (Detail) - USD ($) $ in Thousands		12 Months Ended
		May 31, 2018	May 31, 2017	May 31, 2016
Investments in and Advances to Affiliates [Line Items]
Net sales		$ 1,692,019	$ 1,547,787	$ 1,498,720
Gross margin (loss)		343,281	374,044	345,613
Operating income (loss)		222,893	263,813	236,555
Depreciation and amortization		23,706	26,106	29,228
Interest expense (income)		10,208	8,720	8,469
Income tax expense (benefit)		5,468	14,134	6,577
Net earnings (loss)		206,976	250,748	258,106
WAVE
Investments in and Advances to Affiliates [Line Items]
Net sales		360,395	334,031	329,050
Gross margin (loss)		201,581	190,350	191,535
Operating income (loss)		158,697	158,030	162,026
Depreciation and amortization		1,659	2,978	2,245
Interest expense (income)		8,365	7,182	6,635
Income tax expense (benefit)		119	2,398	(14)
Net earnings (loss)	[1]	152,329	154,866	164,132
ClarkDietrich
Investments in and Advances to Affiliates [Line Items]
Net sales		790,887	711,735	615,609
Gross margin (loss)		97,437	128,098	95,427
Operating income (loss)		39,153	68,696	33,897
Depreciation and amortization		11,864	12,718	14,289
Interest expense (income)		114	20	80
Net earnings (loss)		39,138	69,122	58,539
Serviacero Worthington
Investments in and Advances to Affiliates [Line Items]
Net sales		315,098	275,315	260,337
Gross margin (loss)		32,396	37,080	15,328
Operating income (loss)		24,232	29,975	11,110
Depreciation and amortization		3,919	3,862	3,508
Interest expense (income)		397	89	114
Income tax expense (benefit)		5,141	11,740	6,249
Net earnings (loss)		17,577	18,140	6,246
ArtiFlex
Investments in and Advances to Affiliates [Line Items]
Net sales		197,061	208,922	219,510
Gross margin (loss)		18,266	22,829	30,181
Operating income (loss)		11,395	15,519	22,612
Depreciation and amortization		5,515	5,850	6,105
Interest expense (income)		1,333	1,429	1,650
Income tax expense (benefit)		208	(2)	289
Net earnings (loss)		9,854	14,092	20,673
Other [Member]
Investments in and Advances to Affiliates [Line Items]
Net sales		28,578	17,784	74,214
Gross margin (loss)		(6,399)	(4,313)	13,142
Operating income (loss)		(10,584)	(8,407)	6,910
Depreciation and amortization		749	698	3,081
Interest expense (income)		(1)		(10)
Income tax expense (benefit)			(2)	53
Net earnings (loss)		$ (11,922)	$ (5,472)	$ 8,516

[1] On November 20, 2017, WAVE agreed to sell its business and operations in Europe and Asia to the Knauf Group, a family-owned manufacturer of building materials headquartered in Germany. WAVE has classified its businesses to be sold as discontinued operations. These net earnings include net income attributable to discontinued operations of $2,226,000, $6,775,000, and $8,930,000 in fiscal 2018, fiscal 2017, and fiscal 2016, respectively. All other amounts presented in the table above exclude the activity of the discontinued operations of WAVE