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