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Business segments (Revenue by Shipping Location) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 147,203	$ 138,268	$ 142,723	$ 159,441	$ 196,148	$ 244,993	$ 284,415	$ 348,096	$ 587,635	$ 1,073,652	$ 1,739,717
Sales
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales									$ 587,635	$ 1,073,652	$ 1,739,717
Percentage of net sales									100.00%	100.00%	100.00%
Sales | United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales									$ 361,941	$ 646,928	$ 1,049,609
Percentage of net sales									61.70%	60.30%	60.30%
Sales | Europe & Africa
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales									$ 77,847	$ 188,414	$ 305,376
Percentage of net sales									13.20%	17.50%	17.60%
Sales | Asia-Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales									$ 51,880	$ 69,923	$ 154,669
Percentage of net sales									8.80%	6.50%	8.90%
Sales | Middle East
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales									$ 25,975	$ 59,680	$ 65,498
Percentage of net sales									4.40%	5.60%	3.80%
Sales | Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales									$ 42,520	$ 57,837	$ 103,077
Percentage of net sales									7.20%	5.40%	5.90%
Sales | Latin America
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales									$ 27,472	$ 50,870	$ 61,488
Percentage of net sales									4.70%	4.70%	3.50%

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