Operating segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Feb. 29, 2016	Nov. 30, 2015	Aug. 31, 2015	May. 31, 2015	Feb. 28, 2015	Nov. 30, 2014	Aug. 31, 2014	May. 31, 2014	Feb. 29, 2016	Feb. 28, 2015	Feb. 28, 2014
Operations and assets by segment
Net sales	$ 217,611	$ 242,447	$ 214,246	$ 228,888	$ 182,312	$ 224,833	$ 193,416	$ 216,126	$ 903,192	$ 816,687	$ 751,723
Segment Operating income									122,288	106,825	100,114
Gain (Loss) on Disposition of Property Plant Equipment									(327)	(2,525)	(8,039)
Other Nonoperating Income (Expense)									(3,092)	(2,659)	4,165
Interest expense									15,155	16,561	18,407
Total Operating Income									104,368	90,130	93,911
Depreciation and amortization									47,417	46,089	43,305
Expenditures for acquisitions, net of cash, and property, plant and equipment									100,445	40,895	319,174
Assets	983,371				936,914				983,371	936,914	953,253
Goodwill	292,527				279,074				292,527	279,074	278,556
Property, Plant and Equipment, Net	226,333				196,583				226,333	196,583	197,639
Galvanizing Services [Member]
Operations and assets by segment
Net sales									402,362	358,348	335,617
Segment Operating income									94,766	88,562	87,808
Depreciation and amortization									26,863	23,964	22,008
Expenditures for acquisitions, net of cash, and property, plant and equipment									86,724	26,928	33,282
Assets	436,471				378,823				436,471	378,823	378,358
Goodwill	109,314				95,538				109,314	95,538	94,731
Corporate, Non-Segment [Member]
Operations and assets by segment
Segment Operating income									(30,949)	(20,440)	(32,207)
Corporate [Member]
Operations and assets by segment
Depreciation and amortization									1,423	1,400	1,338
Expenditures for acquisitions, net of cash, and property, plant and equipment									858	3,320	1,378
Assets	46,822				34,844				46,822	34,844	32,086
Energy [Member]
Operations and assets by segment
Net sales									500,830	458,339	416,106
Segment Operating income									58,471	38,703	44,513
Depreciation and amortization									19,131	20,725	19,959
Expenditures for acquisitions, net of cash, and property, plant and equipment									12,863	10,647	284,514
Assets	500,078				523,247				500,078	523,247	542,809
Goodwill	183,213				183,536				183,213	183,536	183,825
UNITED STATES
Operations and assets by segment
Net sales									724,559	631,544	601,674
Property, Plant and Equipment, Net	204,587				173,712				204,587	173,712	171,727
International [Member]
Operations and assets by segment
Net sales									179,832	189,855	150,049
Geography Eliminations [Member]
Operations and assets by segment
Net sales									(1,199)	(4,712)	0
CANADA
Operations and assets by segment
Property, Plant and Equipment, Net	17,868				20,289				17,868	20,289	23,779
Other Countries [Member]
Operations and assets by segment
Property, Plant and Equipment, Net	$ 3,878				$ 2,582				$ 3,878	$ 2,582	$ 2,133