Operating segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	May 31, 2016	Feb. 29, 2016	Nov. 30, 2015	Aug. 31, 2015	May 31, 2015	Feb. 28, 2017	Feb. 29, 2016	Feb. 28, 2015
Operations and assets by segment
Net sales	$ 184,266	$ 228,116	$ 200,790	$ 250,366	$ 207,694	$ 241,596	$ 199,670	$ 240,440	$ 863,538	$ 889,400	$ 819,692
Segment Operating income									98,908	120,295	107,902
Gain (Loss) on Disposition of Property Plant Equipment									76	(327)	(2,525)
Other Nonoperating Income (Expense)									1,197	(3,092)	(2,659)
Interest expense									14,732	15,155	16,561
Total Operating Income									85,297	102,375	91,207
Depreciation and amortization									50,357	47,417	46,089
Expenditures for acquisitions, net of cash, and property, plant and equipment									64,113	100,445	40,895
Assets	978,354				988,201				978,354	988,201	925,315
Goodwill	306,579				292,527				306,579	292,527	279,074
Property, Plant and Equipment, Net	228,610				226,333				228,610	226,333	196,583
Galvanizing Services [Member]
Operations and assets by segment
Net sales									375,536	402,362	358,348
Segment Operating income									79,033	94,766	88,562
Depreciation and amortization									28,650	26,863	23,964
Expenditures for acquisitions, net of cash, and property, plant and equipment									32,099	86,724	26,928
Assets	428,330				436,471				428,330	436,471	378,823
Goodwill	109,980				109,314				109,980	109,314	95,538
Corporate, Non-Segment [Member]
Operations and assets by segment
Segment Operating income									(32,702)	(30,949)	(20,440)
Corporate [Member]
Operations and assets by segment
Depreciation and amortization									2,083	1,423	1,400
Expenditures for acquisitions, net of cash, and property, plant and equipment									540	858	3,320
Assets	13,467				45,461				13,467	45,461	34,844
Energy [Member]
Operations and assets by segment
Net sales									488,002	487,038	461,344
Segment Operating income									52,577	56,478	39,780
Depreciation and amortization									19,624	19,131	20,725
Expenditures for acquisitions, net of cash, and property, plant and equipment									31,474	12,863	10,647
Assets	536,557				506,269				536,557	506,269	511,648
Goodwill	196,599				183,213				196,599	183,213	183,536
UNITED STATES
Operations and assets by segment
Net sales									705,976	710,767	634,549
Property, Plant and Equipment, Net	205,079				204,587				205,079	204,587	173,712
International [Member]
Operations and assets by segment
Net sales									157,718	179,832	189,855
Geography Eliminations [Member]
Operations and assets by segment
Net sales									(156)	(1,199)	(4,712)
CANADA
Operations and assets by segment
Property, Plant and Equipment, Net	18,002				17,868				18,002	17,868	20,289
Other Countries [Member]
Operations and assets by segment
Property, Plant and Equipment, Net	$ 5,529				$ 3,878				$ 5,529	$ 3,878	$ 2,582