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Condensed consolidating financial information - Statement of Cash Flows (Details) - USD ($)
$ in Thousands
 12 Months Ended
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