Segment Operations - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Nov. 30, 2021		Nov. 30, 2020		Nov. 30, 2021		Nov. 30, 2020		May 31, 2021
Segment Reporting Information [Line Items]
Net sales		$ 1,232,861		$ 731,092		$ 2,343,679		$ 1,434,001
Impairment of long-lived assets		0		3,815		0		13,739
Restructuring and other (income) expense, net		(2,004)		7,596		(14,278)		9,444
Equity in net income of unconsolidated affiliates		60,218		25,631		113,134		49,265
Adjusted earnings (loss) before interest and taxes		144,411		(87,957)		326,036		694,559
Total assets		3,517,632				3,517,632				$ 3,373,245
Steel Processing
Segment Reporting Information [Line Items]
Net sales		937,842		468,723		1,760,652		899,743
Restructuring and other (income) expense, net		(182)	[1]	375	[1]	(12,313)	[2]	1,846	[2]
Miscellaneous income (loss), net		17		(5)		47		(48)
Equity in net income of unconsolidated affiliates		8,823		1,861		18,172		3,170
Adjusted earnings (loss) before interest and taxes		71,925		34,381		179,617		48,557
Total assets		1,858,313				1,858,313				1,359,598
Consumer products
Segment Reporting Information [Line Items]
Net sales		140,793		117,513		288,576		251,135
Impairment of long-lived assets								506
Restructuring and other (income) expense, net				120	[1]			120	[2]
Miscellaneous income (loss), net		159		(96)		209		(117)
Adjusted earnings (loss) before interest and taxes		17,584		17,432		38,140		41,341
Total assets		547,132				547,132				541,028
Building Products
Segment Reporting Information [Line Items]
Net sales		121,125		93,989		235,868		182,092
Impairment of long-lived assets								1,423
Miscellaneous income (loss), net		218		70		144		(92)
Equity in net income of unconsolidated affiliates		49,894		22,692		92,887		45,243
Adjusted earnings (loss) before interest and taxes		54,718		25,964		103,471		49,337
Total assets		615,755				615,755				664,113
Sustainable Energy Solutions
Segment Reporting Information [Line Items]
Net sales		33,101		34,023		58,583		61,880
Restructuring and other (income) expense, net	[2]					(143)
Miscellaneous income (loss), net		82		69		22		151
Adjusted earnings (loss) before interest and taxes		796		1,534		(1,760)		973
Total assets		118,834				118,834				169,550
Other
Segment Reporting Information [Line Items]
Net sales				16,844				39,151
Impairment of long-lived assets				3,815				11,810
Restructuring and other (income) expense, net		(1,822)	[1]	7,101	[1]	(1,822)	[2]	7,478	[2]
Incremental expenses related to Nikola gains				4,570				54,081
Miscellaneous income (loss), net		564		338		1,248		933
Equity in net income of unconsolidated affiliates		1,501		1,078		2,075		852
Adjusted earnings (loss) before interest and taxes		1,893		(5,633)		1,479		(13,461)
Total assets		377,598				377,598				$ 638,956
Consolidated
Segment Reporting Information [Line Items]
Net sales		1,232,861		731,092		2,343,679		1,434,001
Impairment of long-lived assets				3,815				13,739
Restructuring and other (income) expense, net		(2,004)	[1]	7,596	[1]	(14,278)	[2]	9,444	[2]
Incremental expenses related to Nikola gains				4,570				54,081
Miscellaneous income (loss), net		1,040		376		1,670		827
Equity in net income of unconsolidated affiliates		60,218		25,631		113,134		49,265
Adjusted earnings (loss) before interest and taxes		$ 146,916		$ 73,678		$ 320,947		$ 126,747

[1] Includes the noncontrolling interest portion of restructuring (charges) gains of $81 and $(142) for the three months ended November 30, 2021 and 2020, respectively

[2] Includes the noncontrolling interest portion of restructuring (charges) gains of $6,027 and $(257) for the six months ended November 30, 2021 and 2020, respectively