Accounting Policies - VIEs, Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Current assets	$ 42,732	$ 51,354
Non-current assets	26,346	25,607
Current liabilities	12,449	29,324
Non-current liabilities	30,675	$ 28,849
Bayou Perma-Pipe Canada, Ltd | Disposed of by Sale
Variable Interest Entity [Line Items]
Total assets held for sale	25,400
Total liabilities held for sale	$ 9,800