Accounting Policies - VIE Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Current assets (VIE)	$ 41,877	$ 42,732
Non-current assets (VIE)	26,725	26,346
Current liabilities (VIE)	12,275	12,449
Non-current liabilities (VIE)	28,371	30,675
Bayou Perma-Pipe Canada, Ltd | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Variable Interest Entity [Line Items]
Disposal group, assets	25,200	25,400
Disposal group, liabilities	$ 9,300	$ 9,800