Condensed consolidating financial information - Statement of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
NET SALES
Related parties	$ 305.3	$ 299.2	$ 884.2	$ 876.1
Other customers	176.5	101.4	522.1	279.1
Total net sales	481.8	400.6	1,406.3	1,155.2
Cost of goods sold	493.6	359.2	1,369.9	1,074.5
Gross profit (loss)	(11.8)	41.4	36.4	80.7
Selling, general and administrative expenses	8.8	14.0	31.5	34.0
Ravenswood (gains) losses	0.0	(5.5)	0.0	(5.5)
Helguvik (gains) losses	(4.5)	0.0	(4.5)	0.0
Other operating (income) expense - net	(0.5)	0.4	0.0	1.6
Operating income (loss)	(15.6)	32.5	9.4	50.6
Interest expense	(5.6)	(5.5)	(16.7)	(16.6)
Intercompany interest	0.0	0.0	0.0	0.0
Interest income	0.4	0.4	1.3	0.9
Net gain (loss) on forward and derivative contracts	0.8	(3.9)	2.8	(17.1)
Other income (expense) - net	0.7	0.4	1.8	(1.0)
Income (loss) before income taxes and equity in earnings of joint ventures	(19.3)	23.9	(1.4)	16.8
Income tax (expense) benefit	(1.7)	(3.3)	(3.0)	(4.5)
Income (loss) before equity in earnings of joint ventures	(21.0)	20.6	(4.4)	12.3
Equity in earnings (loss) of joint ventures	0.7	0.2	3.2	0.5
Net income (loss)	(20.3)	20.8	(1.2)	12.8
Other comprehensive income (loss) before income tax effect	0.5	1.4	4.2	3.1
Income tax effect	(0.4)	(0.4)	(1.1)	(1.2)
Other comprehensive income	0.1	1.0	3.1	1.9
Total comprehensive income (loss)	(20.2)	21.8	1.9	14.7
Consolidating Adjustments [Member]
NET SALES
Related parties	0.0	0.0	0.0	0.0
Other customers	0.0	0.0	0.0	0.0
Total net sales	0.0	0.0	0.0	0.0
Cost of goods sold	0.0	0.0	0.0	0.0
Gross profit (loss)	0.0	0.0	0.0	0.0
Selling, general and administrative expenses	0.0	0.0	0.0	0.0
Ravenswood (gains) losses		0.0		0.0
Helguvik (gains) losses	0.0		0.0
Other operating (income) expense - net	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Intercompany interest	0.0	0.0	0.0	0.0
Interest income	0.0	0.0	0.0	0.0
Net gain (loss) on forward and derivative contracts	0.0	0.0	0.0	0.0
Other income (expense) - net	0.0	0.0	0.0	0.0
Income (loss) before income taxes and equity in earnings of joint ventures	0.0	0.0	0.0	0.0
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Income (loss) before equity in earnings of joint ventures	0.0	0.0	0.0	0.0
Equity in earnings (loss) of joint ventures	13.8	(31.9)	(14.9)	(35.6)
Net income (loss)	13.8	(31.9)	(14.9)	(35.6)
Other comprehensive income (loss) before income tax effect	(0.9)	(1.3)	(5.1)	(2.0)
Income tax effect	0.0	0.0	0.0	0.0
Other comprehensive income	(0.9)	(1.3)	(5.1)	(2.0)
Total comprehensive income (loss)	12.9	(33.2)	(20.0)	(37.6)
The Company [Member] | Reportable Legal Entities [Member]
NET SALES
Related parties	0.0	0.0	0.0	0.0
Other customers	0.0	0.0	0.0	0.0
Total net sales	0.0	0.0	0.0	0.0
Cost of goods sold	0.0	0.0	0.0	0.0
Gross profit (loss)	0.0	0.0	0.0	0.0
Selling, general and administrative expenses	8.4	13.7	29.8	32.8
Ravenswood (gains) losses		0.0		0.0
Helguvik (gains) losses	0.0		0.0
Other operating (income) expense - net	0.0	0.0	0.0	0.0
Operating income (loss)	(8.4)	(13.7)	(29.8)	(32.8)
Interest expense	(5.0)	(5.1)	(15.4)	(15.3)
Intercompany interest	9.3	8.9	27.4	27.1
Interest income	0.1	0.2	0.3	0.3
Net gain (loss) on forward and derivative contracts	(0.2)	0.0	(0.2)	0.0
Other income (expense) - net	0.0	0.0	0.6	0.7
Income (loss) before income taxes and equity in earnings of joint ventures	(4.2)	(9.7)	(17.1)	(20.0)
Income tax (expense) benefit	0.5	0.0	0.7	1.1
Income (loss) before equity in earnings of joint ventures	(3.7)	(9.7)	(16.4)	(18.9)
Equity in earnings (loss) of joint ventures	(16.6)	30.5	15.2	31.7
Net income (loss)	(20.3)	20.8	(1.2)	12.8
Other comprehensive income (loss) before income tax effect	0.5	1.4	4.2	3.1
Income tax effect	(0.4)	(0.4)	(1.1)	(1.2)
Other comprehensive income	0.1	1.0	3.1	1.9
Total comprehensive income (loss)	(20.2)	21.8	1.9	14.7
Combined Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
NET SALES
Related parties	110.8	138.2	315.1	404.1
Other customers	175.8	101.4	519.1	279.1
Total net sales	286.6	239.6	834.2	683.2
Cost of goods sold	301.5	220.7	821.1	655.0
Gross profit (loss)	(14.9)	18.9	13.1	28.2
Selling, general and administrative expenses	0.0	0.0	0.0	0.0
Ravenswood (gains) losses		0.0		0.0
Helguvik (gains) losses	0.0		0.0
Other operating (income) expense - net	0.0	0.0	0.0	0.0
Operating income (loss)	(14.9)	18.9	13.1	28.2
Interest expense	(0.4)	(0.4)	(1.2)	(1.2)
Intercompany interest	2.4	2.2	7.0	6.4
Interest income	0.0	0.0	0.0	0.0
Net gain (loss) on forward and derivative contracts	0.4	(3.9)	1.1	(17.1)
Other income (expense) - net	0.0	0.0	(0.2)	0.2
Income (loss) before income taxes and equity in earnings of joint ventures	(12.5)	16.8	19.8	16.5
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Income (loss) before equity in earnings of joint ventures	(12.5)	16.8	19.8	16.5
Equity in earnings (loss) of joint ventures	2.8	1.4	(0.3)	3.9
Net income (loss)	(9.7)	18.2	19.5	20.4
Other comprehensive income (loss) before income tax effect	0.5	1.0	3.9	0.8
Income tax effect	0.0	0.0	0.0	0.0
Other comprehensive income	0.5	1.0	3.9	0.8
Total comprehensive income (loss)	(9.2)	19.2	23.4	21.2
Combined Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
NET SALES
Related parties	194.5	161.0	569.1	472.0
Other customers	0.7	0.0	3.0	0.0
Total net sales	195.2	161.0	572.1	472.0
Cost of goods sold	192.1	138.5	548.8	419.5
Gross profit (loss)	3.1	22.5	23.3	52.5
Selling, general and administrative expenses	0.4	0.3	1.7	1.2
Ravenswood (gains) losses		(5.5)		(5.5)
Helguvik (gains) losses	(4.5)		(4.5)
Other operating (income) expense - net	(0.5)	0.4	0.0	1.6
Operating income (loss)	7.7	27.3	26.1	55.2
Interest expense	(0.2)	0.0	(0.1)	(0.1)
Intercompany interest	(11.7)	(11.1)	(34.4)	(33.5)
Interest income	0.3	0.2	1.0	0.6
Net gain (loss) on forward and derivative contracts	0.6	0.0	1.9	0.0
Other income (expense) - net	0.7	0.4	1.4	(1.9)
Income (loss) before income taxes and equity in earnings of joint ventures	(2.6)	16.8	(4.1)	20.3
Income tax (expense) benefit	(2.2)	(3.3)	(3.7)	(5.6)
Income (loss) before equity in earnings of joint ventures	(4.8)	13.5	(7.8)	14.7
Equity in earnings (loss) of joint ventures	0.7	0.2	3.2	0.5
Net income (loss)	(4.1)	13.7	(4.6)	15.2
Other comprehensive income (loss) before income tax effect	0.4	0.3	1.2	1.2
Income tax effect	0.0	0.0	0.0	0.0
Other comprehensive income	0.4	0.3	1.2	1.2
Total comprehensive income (loss)	$ (3.7)	$ 14.0	$ (3.4)	$ 16.4
Century Aluminum [Member] | Combined Guarantor Subsidiaries [Member]
Condensed Statement of Income Captions [Line Items]
Ownership percentage	100.00%		100.00%