Operating segments (Details) - USD ($) $ in Thousands	3 Months Ended
	May 31, 2024	May 31, 2023	Feb. 29, 2024	Sep. 30, 2022
Operations and assets by segment
Sales	$ 413,208	$ 390,873
Cost of sales	310,538	293,854
Gross margin	102,670	97,019
Selling, general and administrative	32,921	31,523
Operating income (loss)	69,749	65,496
Interest expense, net	(22,774)	(28,706)
Equity in earnings of unconsolidated subsidiaries	3,824	1,420
Other income	204	(38)
Income (loss) before income tax	51,003	38,172
Income taxes	11,401	9,650
Net income from continuing operations	39,602	28,522
Assets:	2,235,779		$ 2,195,505
Property, plant and equipment, net	$ 555,355		541,652
AIS Joint Venture
Operations and assets by segment
Ownership percentage of investment	40.00%			40.00%
United States
Operations and assets by segment
Sales	$ 403,051	381,322
Property, plant and equipment, net	536,857		522,693
Canada
Operations and assets by segment
Sales	10,157	9,551
Property, plant and equipment, net	18,498		18,959
Corporate
Operations and assets by segment
Sales	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	18,248	17,643
Operating income (loss)	(18,248)	(17,643)
Interest expense, net	(22,774)	(28,706)
Equity in earnings of unconsolidated subsidiaries	0	0
Other income	148	(14)
Income (loss) before income tax	(40,874)	(46,363)
Income taxes	11,401	9,650
Net income from continuing operations	(52,275)	(56,013)
Assets:	49,185		43,709
Metal Coatings | Operating Segments
Operations and assets by segment
Sales	176,651	168,794
Cost of sales	122,735	117,858
Gross margin	53,916	50,936
Selling, general and administrative	5,984	5,466
Operating income (loss)	47,932	45,470
Interest expense, net	0	0
Equity in earnings of unconsolidated subsidiaries	0	0
Other income	56	(24)
Income (loss) before income tax	47,988	45,446
Assets:	550,741		553,505
Precoat Metals | Operating Segments
Operations and assets by segment
Sales	236,557	222,079
Cost of sales	187,803	175,996
Gross margin	48,754	46,083
Selling, general and administrative	8,660	8,392
Operating income (loss)	40,094	37,691
Interest expense, net	0	0
Equity in earnings of unconsolidated subsidiaries	0	0
Other income	0	0
Income (loss) before income tax	40,094	37,691
Assets:	1,534,214		1,500,122
Infrastructure Solutions | Operating Segments
Operations and assets by segment
Sales	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	29	22
Operating income (loss)	(29)	(22)
Interest expense, net	0	0
Equity in earnings of unconsolidated subsidiaries	3,824	1,420
Other income	0	0
Income (loss) before income tax	3,795	$ 1,398
Assets:	$ 101,639		$ 98,169