Segment Information - Reconciliation of Adjusted EBITDA to Income Before Provision for Income Taxes and Equity Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Net sales	$ 378.6	$ 349.1	$ 1,149.3	$ 1,015.9
Depreciation expense	(4.5)	(3.7)	(13.5)	(11.0)
Amortization expense	(0.1)	(0.1)	(0.1)	(0.1)
Interest expense	(4.4)	(5.2)	(13.9)	(21.5)
Interest income	0.2	0.0	0.3	0.0
Net income attributable to noncontrolling interest	0.1	0.2	0.4	0.4
Unrealized (loss) gain on derivative contracts (a)	0.3	(0.4)	(1.1)	2.2
Refinancing costs (b)	(0.9)	(20.1)	(0.9)	(23.4)
Specified legal / professional expenses (c)	(0.5)	(0.5)	(0.5)	(1.1)
Lower of cost or market adjustment to inventory (d)	0.7	2.2	0.8	2.1
Share-based compensation expense (e)	(2.0)	(2.2)	(6.3)	(4.9)
Income before provision for income taxes	19.2	4.8	66.3	36.9
Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	17.6	17.0	61.8	55.4
Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	33.9	39.5	105.7	107.5
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	(3.6)	(4.9)	(4.6)	(13.3)
Olin Brass [Member]
Segment Reporting Information [Line Items]
Net sales	156.9	152.9	483.0	421.9
Olin Brass [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	17.6	16.8	61.6	54.6
Olin Brass [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	12.0	18.6	38.7	39.7
Chase Brass [Member]
Segment Reporting Information [Line Items]
Net sales	144.8	124.1	439.8	381.5
Chase Brass [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	0.0	0.2	0.1	0.8
Chase Brass [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	18.3	16.2	56.6	53.4
A.J. Oster [Member]
Segment Reporting Information [Line Items]
Net sales	76.9	72.1	226.5	212.5
A.J. Oster [Member] | Intersegment [Member]
Segment Reporting Information [Line Items]
Net sales	0.0	0.0	0.1	0.0
A.J. Oster [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 3.6	$ 4.7	$ 10.4	$ 14.4