Condensed consolidating financial information - Cash Flows (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	$ 15,582	$ 32,509
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	(13,127)	(47,595)
Purchase of remaining interest in Mt. Holly smelter	0	11,313
Restricted and other cash deposits	(255)	(141)
Net cash used in investing activities	(13,382)	(36,423)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities	900	1,414
Repayments under revolving credit facilities	(900)	(1,414)
Repurchase of common stock	0	(36,352)
Intercompany transactions	0	0
Net cash used in financing activities	0	(36,352)
CHANGE IN CASH AND CASH EQUIVALENTS	2,200	(40,266)
Cash and cash equivalents, beginning of period	115,393	163,242
Cash and cash equivalents, end of period	117,593	122,976
Consolidating Adjustments [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	15,985	180,347
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	0	(440)
Purchase of remaining interest in Mt. Holly smelter		0
Restricted and other cash deposits	0	0
Net cash used in investing activities	0	(440)
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	(15,985)	(179,907)
Net cash used in financing activities	(15,985)	(179,907)
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	(20,816)	43,577
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	0	(3,513)
Purchase of remaining interest in Mt. Holly smelter		0
Restricted and other cash deposits	0	0
Net cash used in investing activities	0	(3,513)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Borrowings under revolving credit facilities	900	1,414
Repayments under revolving credit facilities	(900)	(1,414)
Repurchase of common stock		(36,352)
Intercompany transactions	0	0
Net cash used in financing activities	0	(36,352)
CHANGE IN CASH AND CASH EQUIVALENTS	(20,816)	3,712
Cash and cash equivalents, beginning of period	58,421	70,683
Cash and cash equivalents, end of period	37,605	74,395
Combined Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	23,044	6,535
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	(4,252)	(23,555)
Purchase of remaining interest in Mt. Holly smelter		11,313
Restricted and other cash deposits	1	(155)
Net cash used in investing activities	(4,251)	(12,397)
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	(15,271)	6,433
Net cash used in financing activities	(15,271)	6,433
CHANGE IN CASH AND CASH EQUIVALENTS	3,522	571
Cash and cash equivalents, beginning of period	(3,647)	(1,187)
Cash and cash equivalents, end of period	(125)	(616)
Combined Non-Guarantor Subsidiaries [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net cash provided by (used in) operating activities	(2,631)	(197,950)
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchase of property, plant and equipment	(8,875)	(20,087)
Purchase of remaining interest in Mt. Holly smelter		0
Restricted and other cash deposits	(256)	14
Net cash used in investing activities	(9,131)	(20,073)
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	31,256	173,474
Net cash used in financing activities	31,256	173,474
CHANGE IN CASH AND CASH EQUIVALENTS	19,494	(44,549)
Cash and cash equivalents, beginning of period	60,619	93,746
Cash and cash equivalents, end of period	$ 80,113	$ 49,197

Payments for (Proceeds from) Acquisition of Remaining Interest in Business, Gross

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