Condensed consolidating financial information - Cash Flows (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	$ 50,667	$ 15,327
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(23,590)	(13,127)
Proceeds from sale of property, plant and equipment	14,452	0
Intercompany transactions	0
Net cash used in investing activities	(9,138)	(13,127)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	1,022	900
Repayments under revolving credit facilities	(1,022)	(900)
Issuance of common stock	281	0
Intercompany transactions	0	0
Net cash provided by financing activities	281	0
CHANGE IN CASH AND CASH EQUIVALENTS	41,810	2,200
Cash and cash equivalents, beginning of period	132,403	115,393
Cash and cash equivalents, end of period	174,213	117,593
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	0	15,985
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	0	0
Proceeds from sale of property, plant and equipment	0
Intercompany transactions	(614)
Net cash used in investing activities	(614)	0
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0	0
Repayments under revolving credit facilities	0	0
Issuance of common stock	0
Intercompany transactions	614	(15,985)
Net cash provided by financing activities	614	(15,985)
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] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(25,306)	(20,816)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(8,773)	0
Proceeds from sale of property, plant and equipment	0
Intercompany transactions	34,249
Net cash used in investing activities	25,476	0
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	1,022	900
Repayments under revolving credit facilities	(1,022)	(900)
Issuance of common stock	281
Intercompany transactions	33,631	0
Net cash provided by financing activities	33,912	0
CHANGE IN CASH AND CASH EQUIVALENTS	34,082	(20,816)
Cash and cash equivalents, beginning of period	36,670	58,421
Cash and cash equivalents, end of period	70,752	37,605
Combined Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	14,298	23,045
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(5,880)	(4,252)
Proceeds from sale of property, plant and equipment	831
Intercompany transactions	(25,996)
Net cash used in investing activities	(31,045)	(4,252)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0	0
Repayments under revolving credit facilities	0	0
Issuance of common stock	0
Intercompany transactions	16,785	(15,271)
Net cash provided by financing activities	16,785	(15,271)
CHANGE IN CASH AND CASH EQUIVALENTS	38	3,522
Cash and cash equivalents, beginning of period	(255)	(3,647)
Cash and cash equivalents, end of period	(217)	(125)
Combined Non-Guarantor Subsidiaries [Member] \| Reportable Legal Entities [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	61,675	(2,887)
Net cash provided by (used in) investing activities
Purchase of property, plant and equipment	(8,937)	(8,875)
Proceeds from sale of property, plant and equipment	13,621
Intercompany transactions	(7,639)
Net cash used in investing activities	(2,955)	(8,875)
Net cash provided by (used in) financing activities
Borrowings under revolving credit facilities	0	0
Repayments under revolving credit facilities	0	0
Issuance of common stock	0
Intercompany transactions	(51,030)	31,256
Net cash provided by financing activities	(51,030)	31,256
CHANGE IN CASH AND CASH EQUIVALENTS	7,690	19,494
Cash and cash equivalents, beginning of period	95,988	60,619
Cash and cash equivalents, end of period	$ 103,678	$ 80,113

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