Condensed Consolidating Financial Information - Statement of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
NET SALES
Related parties	$ 305.0	$ 282.7	$ 616.4	$ 579.0
Other customers	168.1	187.3	346.8	345.5
Total net sales	473.1	470.0	963.2	924.5
Cost of goods sold	477.2	436.3	980.1	876.2
Gross profit (loss)	(4.1)	33.7	(16.9)	48.3
Selling, general and administrative expenses	11.9	12.0	26.6	22.7
Other operating expense, net	0.3	0.2	0.6	0.6
Operating income (loss)	(16.3)	21.5	(44.1)	25.0
Interest expense - term loan	(0.6)	0.0	(0.6)	0.0
Interest expense	(5.8)	(5.6)	(11.7)	(11.2)
Intercompany interest	0.0	0.0	0.0	0.0
Interest income	0.2	0.5	0.4	0.9
Net gain (loss) on forward and derivative contracts	6.1	1.2	0.4	1.9
Other income (expense) - net	(1.7)	2.1	(0.5)	1.3
Income (loss) before income taxes and equity in earnings of joint ventures	(18.1)	19.7	(56.1)	17.9
Income tax benefit (expense)	1.6	(2.3)	4.4	(1.4)
Income (loss) before equity in earnings of joint ventures	(16.5)	17.4	(51.7)	16.5
Loss on sale of BHH	(4.3)	0.0	(4.3)	0.0
Equity in earnings of joint ventures	0.1	2.0	0.7	2.6
Net income (loss)	(20.7)	19.4	(55.3)	19.1
Other comprehensive income before income tax effect	1.2	3.0	1.8	3.8
Income tax effect	(0.2)	(0.4)	(0.5)	(0.8)
Other comprehensive income	1.0	2.6	1.3	3.0
Total comprehensive income (loss)	(19.7)	22.0	(54.0)	22.1
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
Other operating expense, net	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Interest expense - term loan	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 benefit (expense)	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
Loss on sale of BHH	0.0		0.0
Equity in earnings of joint ventures	17.4	(23.8)	38.0	(28.6)
Net income (loss)	17.4	(23.8)	38.0	(28.6)
Other comprehensive income before income tax effect	(1.5)	(3.1)	(2.4)	(4.3)
Income tax effect	0.0	0.0	0.0	0.0
Other comprehensive income	(1.5)	(3.1)	(2.4)	(4.3)
Total comprehensive income (loss)	15.9	(26.9)	35.6	(32.9)
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	11.5	11.4	25.6	21.4
Other operating expense, net	0.0	0.0	0.0	0.0
Operating income (loss)	(11.5)	(11.4)	(25.6)	(21.4)
Interest expense - term loan	(0.6)		(0.6)
Interest expense	(5.3)	(5.1)	(10.8)	(10.3)
Intercompany interest	8.7	9.0	17.4	18.1
Interest income	0.1	0.1	0.1	0.2
Net gain (loss) on forward and derivative contracts	4.8	0.0	0.1	0.0
Other income (expense) - net	0.0	0.0	0.9	0.7
Income (loss) before income taxes and equity in earnings of joint ventures	(3.8)	(7.4)	(18.5)	(12.7)
Income tax benefit (expense)	(0.1)	0.3	0.9	0.1
Income (loss) before equity in earnings of joint ventures	(3.9)	(7.1)	(17.6)	(12.6)
Loss on sale of BHH	0.0		0.0
Equity in earnings of joint ventures	(16.8)	26.5	(37.7)	31.7
Net income (loss)	(20.7)	19.4	(55.3)	19.1
Other comprehensive income before income tax effect	1.2	3.0	1.8	3.8
Income tax effect	(0.2)	(0.4)	(0.5)	(0.8)
Other comprehensive income	1.0	2.6	1.3	3.0
Total comprehensive income (loss)	(19.7)	22.0	(54.0)	22.1
Combined Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
NET SALES
Related parties	147.4	93.6	299.9	204.4
Other customers	162.2	185.8	334.9	343.2
Total net sales	309.6	279.4	634.8	547.6
Cost of goods sold	306.0	258.5	633.2	519.6
Gross profit (loss)	3.6	20.9	1.6	28.0
Selling, general and administrative expenses	0.0	0.0	0.0	0.0
Other operating expense, net	0.0	0.0	0.0	0.0
Operating income (loss)	3.6	20.9	1.6	28.0
Interest expense - term loan	0.0		0.0
Interest expense	(0.4)	(0.4)	(0.8)	(0.8)
Intercompany interest	2.5	2.3	5.0	4.6
Interest income	0.0	0.0	0.0	0.0
Net gain (loss) on forward and derivative contracts	0.3	0.4	0.7	0.7
Other income (expense) - net	(1.3)	(0.1)	(1.9)	(0.2)
Income (loss) before income taxes and equity in earnings of joint ventures	4.7	23.1	4.6	32.3
Income tax benefit (expense)	0.0	0.0	0.0	0.0
Income (loss) before equity in earnings of joint ventures	4.7	23.1	4.6	32.3
Loss on sale of BHH	0.0		0.0
Equity in earnings of joint ventures	(0.6)	(2.7)	(0.3)	(3.1)
Net income (loss)	4.1	20.4	4.3	29.2
Other comprehensive income before income tax effect	0.3	2.8	0.9	3.5
Income tax effect	0.0	0.0	0.0	0.0
Other comprehensive income	0.3	2.8	0.9	3.5
Total comprehensive income (loss)	4.4	23.2	5.2	32.7
Combined Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
NET SALES
Related parties	157.6	189.1	316.5	374.6
Other customers	5.9	1.5	11.9	2.3
Total net sales	163.5	190.6	328.4	376.9
Cost of goods sold	171.2	177.8	346.9	356.6
Gross profit (loss)	(7.7)	12.8	(18.5)	20.3
Selling, general and administrative expenses	0.4	0.6	1.0	1.3
Other operating expense, net	0.3	0.2	0.6	0.6
Operating income (loss)	(8.4)	12.0	(20.1)	18.4
Interest expense - term loan	0.0		0.0
Interest expense	(0.1)	(0.1)	(0.1)	(0.1)
Intercompany interest	(11.2)	(11.3)	(22.4)	(22.7)
Interest income	0.1	0.4	0.3	0.7
Net gain (loss) on forward and derivative contracts	1.0	0.8	(0.4)	1.2
Other income (expense) - net	(0.4)	2.2	0.5	0.8
Income (loss) before income taxes and equity in earnings of joint ventures	(19.0)	4.0	(42.2)	(1.7)
Income tax benefit (expense)	1.7	(2.6)	3.5	(1.5)
Income (loss) before equity in earnings of joint ventures	(17.3)	1.4	(38.7)	(3.2)
Loss on sale of BHH	(4.3)		(4.3)
Equity in earnings of joint ventures	0.1	2.0	0.7	2.6
Net income (loss)	(21.5)	3.4	(42.3)	(0.6)
Other comprehensive income before income tax effect	1.2	0.3	1.5	0.8
Income tax effect	0.0	0.0	0.0	0.0
Other comprehensive income	1.2	0.3	1.5	0.8
Total comprehensive income (loss)	$ (20.3)	$ 3.7	$ (40.8)	$ 0.2
Century Aluminum [Member] | Combined Guarantor Subsidiaries [Member]
Condensed Statement of Income Captions [Line Items]
Ownership percentage	100.00%		100.00%