Condensed consolidating financial information - Cash Flows (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	$ 3,093	$ 23,603
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	(18,843)	(7,734)
Proceeds from sale of property, plant and equipment	13,585	0
Restricted and other cash deposits	195	(1,878)
Intercompany transactions	0
Net cash used in investing activities	(5,063)	(9,612)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities	720	595
Repayments under revolving credit facilities	(720)	(595)
Issuance of common stock	240	0
Intercompany transactions	0	0
Net cash provided by financing activities	240	0
CHANGE IN CASH AND CASH EQUIVALENTS	(1,730)	13,991
Cash and cash equivalents, beginning of period	132,403	115,393
Cash and cash equivalents, end of period	130,673	129,384
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	0	4,593
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	0	0
Proceeds from sale of property, plant and equipment	0
Restricted and other cash deposits	0	0
Intercompany transactions	28,208
Net cash used in investing activities	28,208	0
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities	0	0
Repayments under revolving credit facilities	0	0
Issuance of common stock	0
Intercompany transactions	(28,208)	(4,593)
Net cash provided by financing activities	(28,208)	(4,593)
CHANGE IN CASH AND CASH EQUIVALENTS	0	0
Cash and cash equivalents, beginning of period	0	0
Cash and cash equivalents, end of period	0	0
The Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(7,988)	(11,105)
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	(7,811)	0
Proceeds from sale of property, plant and equipment	0
Restricted and other cash deposits	0	0
Intercompany transactions	11,784
Net cash used in investing activities	3,973	0
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities	720	595
Repayments under revolving credit facilities	(720)	(595)
Issuance of common stock	240
Intercompany transactions	(3,590)	0
Net cash provided by financing activities	(3,350)	0
CHANGE IN CASH AND CASH EQUIVALENTS	(7,365)	(11,105)
Cash and cash equivalents, beginning of period	36,670	58,421
Cash and cash equivalents, end of period	29,305	47,316
Combined Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(25,946)	18,847
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	(4,963)	(2,697)
Proceeds from sale of property, plant and equipment	0
Restricted and other cash deposits	(6)	(4)
Intercompany transactions	(27,931)
Net cash used in investing activities	(32,900)	(2,701)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities	0	0
Repayments under revolving credit facilities	0	0
Issuance of common stock	0
Intercompany transactions	58,831	(12,777)
Net cash provided by financing activities	58,831	(12,777)
CHANGE IN CASH AND CASH EQUIVALENTS	(15)	3,369
Cash and cash equivalents, beginning of period	(255)	(3,647)
Cash and cash equivalents, end of period	(270)	(278)
Combined Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	37,027	11,268
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	(6,069)	(5,037)
Proceeds from sale of property, plant and equipment	13,585
Restricted and other cash deposits	201	(1,874)
Intercompany transactions	(12,061)
Net cash used in investing activities	(4,344)	(6,911)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities	0	0
Repayments under revolving credit facilities	0	0
Issuance of common stock	0
Intercompany transactions	(27,033)	17,370
Net cash provided by financing activities	(27,033)	17,370
CHANGE IN CASH AND CASH EQUIVALENTS	5,650	21,727
Cash and cash equivalents, beginning of period	95,988	60,619
Cash and cash equivalents, end of period	$ 101,638	$ 82,346

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