Condensed consolidating financial information - Cash Flows (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
Apr. 30, 2015	Mar. 31, 2016	Mar. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities		$ 14,903	$ 116,267
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment		(3,835)	(12,627)
Restricted and other cash deposits	$ 21,000	0	(21,012)
Net cash used in investing activities		(3,835)	(33,639)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities		371	455
Repayments under revolving credit facilities		(371)	(455)
Repurchase of common stock		0	(19,439)
Intercompany transactions		0	0
Net cash used in financing activities		0	(19,439)
CHANGE IN CASH AND CASH EQUIVALENTS		11,068	63,189
Cash and cash equivalents, beginning of period	226,431	115,393	163,242
Cash and cash equivalents, end of period		126,461	226,431
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities		8,266	185,624
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment		0	0
Restricted and other cash deposits			0
Net cash used in investing activities		0	0
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities		0	0
Repayments under revolving credit facilities		0	0
Repurchase of common stock			0
Intercompany transactions		(8,266)	(185,624)
Net cash used in financing activities		(8,266)	(185,624)
CHANGE IN CASH AND CASH EQUIVALENTS		0	0
Cash and cash equivalents, beginning of period	0	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		(8,754)	113,179
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment		0	(348)
Restricted and other cash deposits			0
Net cash used in investing activities		0	(348)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities		371	455
Repayments under revolving credit facilities		(371)	(455)
Repurchase of common stock			(19,439)
Intercompany transactions		0	0
Net cash used in financing activities		0	(19,439)
CHANGE IN CASH AND CASH EQUIVALENTS		(8,754)	93,392
Cash and cash equivalents, beginning of period	164,075	58,421	70,683
Cash and cash equivalents, end of period		49,667	164,075
Combined Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities		5,435	40,025
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment		(1,426)	(6,726)
Restricted and other cash deposits			(21,012)
Net cash used in investing activities		(1,426)	(27,738)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities		0	0
Repayments under revolving credit facilities		0	0
Repurchase of common stock			0
Intercompany transactions		(436)	(11,164)
Net cash used in financing activities		(436)	(11,164)
CHANGE IN CASH AND CASH EQUIVALENTS		3,573	1,123
Cash and cash equivalents, beginning of period	(64)	(3,647)	(1,187)
Cash and cash equivalents, end of period		(74)	(64)
Combined Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities		9,956	(222,561)
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment		(2,409)	(5,553)
Restricted and other cash deposits			0
Net cash used in investing activities		(2,409)	(5,553)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities		0	0
Repayments under revolving credit facilities		0	0
Repurchase of common stock			0
Intercompany transactions		8,702	196,788
Net cash used in financing activities		8,702	196,788
CHANGE IN CASH AND CASH EQUIVALENTS		16,249	(31,326)
Cash and cash equivalents, beginning of period	$ 62,420	60,619	93,746
Cash and cash equivalents, end of period		$ 76,868	$ 62,420

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