Investments in Unconsolidated Affiliates - Financial Results of Four Largest Unconsolidated Affiliates (Detail) - USD ($) $ in Thousands		12 Months Ended
		May 31, 2019	May 31, 2018	May 31, 2017
Investments in and Advances to Affiliates [Line Items]
Net sales		$ 1,857,811	$ 1,692,019	$ 1,547,787
Gross margin (loss)		341,278	343,281	374,044
Operating income (loss)		221,549	222,893	263,813
Depreciation and amortization		26,483	25,365	26,106
Interest expense		13,395	10,209	8,720
Income tax expense (benefit)		7,877	5,468	14,134
Net earnings (loss)		209,232	206,976	250,748
WAVE
Investments in and Advances to Affiliates [Line Items]
Net sales		379,103	360,395	334,031
Gross margin (loss)		205,909	201,581	190,350
Operating income (loss)		166,969	158,697	158,030
Depreciation and amortization		3,634	3,318	2,978
Interest expense		10,547	8,365	7,182
Income tax expense (benefit)		219	119	2,398
Net earnings (loss)	[1]	162,849	152,329	154,866
ClarkDietrich
Investments in and Advances to Affiliates [Line Items]
Net sales		892,758	790,887	711,735
Gross margin (loss)		93,947	97,437	128,098
Operating income (loss)		33,384	39,153	68,696
Depreciation and amortization		11,600	11,864	12,718
Interest expense		912	114	20
Net earnings (loss)		34,560	39,138	69,122
Serviacero Worthington
Investments in and Advances to Affiliates [Line Items]
Net sales		351,671	315,098	275,315
Gross margin (loss)		34,494	32,396	37,080
Operating income (loss)		25,636	24,232	29,975
Depreciation and amortization		4,319	3,919	3,862
Interest expense		493	397	89
Income tax expense (benefit)		7,629	5,141	11,740
Net earnings (loss)		16,155	17,577	18,140
ArtiFlex
Investments in and Advances to Affiliates [Line Items]
Net sales		201,526	197,061	208,922
Gross margin (loss)		12,928	18,266	22,829
Operating income (loss)		5,524	11,395	15,519
Depreciation and amortization		6,055	5,515	5,850
Interest expense		1,443	1,333	1,429
Income tax expense (benefit)		29	208	(2)
Net earnings (loss)		4,051	9,854	14,092
Other [Member]
Investments in and Advances to Affiliates [Line Items]
Net sales		32,753	28,578	17,784
Gross margin (loss)		(6,000)	(6,399)	(4,313)
Operating income (loss)		(9,964)	(10,584)	(8,407)
Depreciation and amortization		875	749	698
Income tax expense (benefit)				(2)
Net earnings (loss)		$ (8,383)	$ (11,922)	$ (5,472)

[1] These net earnings include net income attributable to discontinued operations of $6,830,000, $2,226,000, and $6,775,000 in fiscal 2019, fiscal 2018, and fiscal 2017, respectively, related to the international operations of WAVE being sold. All other amounts presented in the table above exclude the activity of the discontinued operations of WAVE.