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Condensed consolidating financial information - Statement of Cash Flows (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES: |  |  |
Net cash provided by operating activities | $ 51,759 | $ 38,173 | $ 31,866
CASH FLOWS FROM INVESTING ACTIVITIES: |  |  |
Purchase of property, plant and equipment | (31,839) | (21,944) | (54,700)
Purchase of remaining interest in Mt. Holly smelter | 0 | 0 | 11,313
Proceeds from sale of property, plant and equipment | 14,484 | 1,040 | 14
Restricted and other cash deposits | 0 | (259) | 10
Intercompany transactions | 0 | 0 | 0
Net cash used in investing activities | (17,355) | (21,163) | (43,363)
CASH FLOWS FROM FINANCING ACTIVITIES: |  |  |
Borrowings under revolving credit facilities | 1,281 | 1,179 | 1,737
Repayments under revolving credit facilities | (1,281) | (1,179) | (1,737)
Repurchase of common stock | 0 | 0 | (36,352)
Issuance of common stock | 404 | 0 | 0
Intercompany transactions | 0 | 0 | 0
Net cash provided by (used in) financing activities | 404 | 0 | (36,352)
CHANGE IN CASH AND CASH EQUIVALENTS | 34,808 | 17,010 | (47,849)
Cash and cash equivalents, beginning of period | 132,403 | 115,393 | 163,242
Cash and cash equivalents, end of period | 167,211 | 132,403 | 115,393
Reportable Legal Entities [Member] | The Company [Member] |  |  |
CASH FLOWS FROM OPERATING ACTIVITIES: |  |  |
Net cash provided by operating activities | (34,417) | (63,446) | 25,423
CASH FLOWS FROM INVESTING ACTIVITIES: |  |  |
Purchase of property, plant and equipment | (10,889) | (1,425) | (8,574)
Purchase of remaining interest in Mt. Holly smelter |  |  | 11,313
Proceeds from sale of property, plant and equipment | 0 | 0 | 0
Restricted and other cash deposits |  | 0 | 0
Intercompany transactions | 75,726 | 27,761 | (4,072)
Net cash used in investing activities | 64,837 | 26,336 | (1,333)
CASH FLOWS FROM FINANCING ACTIVITIES: |  |  |
Borrowings under revolving credit facilities | 1,281 | 1,179 | 1,737
Repayments under revolving credit facilities | (1,281) | (1,179) | (1,737)
Repurchase of common stock |  |  | (36,352)
Issuance of common stock | 404 |  |
Intercompany transactions | (3,110) | 15,359 | 0
Net cash provided by (used in) financing activities | (2,706) | 15,359 | (36,352)
CHANGE IN CASH AND CASH EQUIVALENTS | 27,714 | (21,751) | (12,262)
Cash and cash equivalents, beginning of period | 36,670 | 58,421 | 70,683
Cash and cash equivalents, end of period | 64,384 | 36,670 | 58,421
Reportable Legal Entities [Member] | Combined Guarantor Subsidiaries [Member] |  |  |
CASH FLOWS FROM OPERATING ACTIVITIES: |  |  |
Net cash provided by operating activities | (6,993) | 19,019 | 30,890
CASH FLOWS FROM INVESTING ACTIVITIES: |  |  |
Purchase of property, plant and equipment | (8,111) | (7,807) | (21,477)
Purchase of remaining interest in Mt. Holly smelter |  |  | 0
Proceeds from sale of property, plant and equipment | 899 | 0 | 14
Restricted and other cash deposits |  | (3) | (4)
Intercompany transactions | 6,081 | (15,305) | 0
Net cash used in investing activities | (1,131) | (23,115) | (21,467)
CASH FLOWS FROM FINANCING ACTIVITIES: |  |  |
Borrowings under revolving credit facilities | 0 | 0 | 0
Repayments under revolving credit facilities | 0 | 0 | 0
Repurchase of common stock |  |  | 0
Issuance of common stock | 0 |  |
Intercompany transactions | 8,247 | 7,488 | (11,883)
Net cash provided by (used in) financing activities | 8,247 | 7,488 | (11,883)
CHANGE IN CASH AND CASH EQUIVALENTS | 123 | 3,392 | (2,460)
Cash and cash equivalents, beginning of period | (255) | (3,647) | (1,187)
Cash and cash equivalents, end of period | (132) | (255) | (3,647)
Reportable Legal Entities [Member] | Combined Non-Guarantor Subsidiaries [Member] |  |  |
CASH FLOWS FROM OPERATING ACTIVITIES: |  |  |
Net cash provided by operating activities | 96,123 | 82,600 | 65,085
CASH FLOWS FROM INVESTING ACTIVITIES: |  |  |
Purchase of property, plant and equipment | (12,839) | (12,712) | (24,209)
Purchase of remaining interest in Mt. Holly smelter |  |  | 0
Proceeds from sale of property, plant and equipment | 13,585 | 1,040 | 0
Restricted and other cash deposits |  | (256) | 14
Intercompany transactions | (7,638) | (937) | 0
Net cash used in investing activities | (6,892) | (12,865) | (24,195)
CASH FLOWS FROM FINANCING ACTIVITIES: |  |  |
Borrowings under revolving credit facilities | 0 | 0 | 0
Repayments under revolving credit facilities | 0 | 0 | 0
Repurchase of common stock |  |  | 0
Issuance of common stock | 0 |  |
Intercompany transactions | (82,260) | (34,366) | (74,017)
Net cash provided by (used in) financing activities | (82,260) | (34,366) | (74,017)
CHANGE IN CASH AND CASH EQUIVALENTS | 6,971 | 35,369 | (33,127)
Cash and cash equivalents, beginning of period | 95,988 | 60,619 | 93,746
Cash and cash equivalents, end of period | 102,959 | 95,988 | 60,619
Consolidating Adjustments [Member] |  |  |
CASH FLOWS FROM OPERATING ACTIVITIES: |  |  |
Net cash provided by operating activities | (2,954) | 0 | (89,532)
CASH FLOWS FROM INVESTING ACTIVITIES: |  |  |
Purchase of property, plant and equipment | 0 | 0 | (440)
Purchase of remaining interest in Mt. Holly smelter |  |  | 0
Proceeds from sale of property, plant and equipment | 0 | 0 | 0
Restricted and other cash deposits |  | 0 | 0
Intercompany transactions | (74,169) | (11,519) | 4,072
Net cash used in investing activities | (74,169) | (11,519) | 3,632
CASH FLOWS FROM FINANCING ACTIVITIES: |  |  |
Borrowings under revolving credit facilities | 0 | 0 | 0
Repayments under revolving credit facilities | 0 | 0 | 0
Repurchase of common stock |  |  | 0
Issuance of common stock | 0 |  |
Intercompany transactions | 77,123 | 11,519 | 85,900
Net cash provided by (used in) financing activities | 77,123 | 11,519 | 85,900
CHANGE IN CASH AND CASH EQUIVALENTS | 0 | 0 | 0
Cash and cash equivalents, beginning of period | 0 | 0 | 0
Cash and cash equivalents, end of period | $ 0 | $ 0 | $ 0

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