Segment and Geographical Area Information by Operating Segment Table (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Net sales:
Net Sales	$ 415.4	$ 356.3	$ 803.4	$ 711.6
Segment operating income (loss):
Operating income	34.7	12.5	71.8	71.8
Interest expense	(5.7)	(5.5)	(11.3)	(11.1)
Other (expense) income, net	(0.5)	(0.1)	(0.4)	0.7
Income before income taxes	28.5	6.9	60.1	61.4
Depreciation and amortization:
Depreciation and amortization	10.9	9.5	21.4	19.1
Capital expenditures:
Capital expenditures	16.1	24.9	35.8	39.7
Assets:
Assets	1,423.8		1,423.8		$ 1,396.8	$ 1,385.2
Fabricated Products
Segment operating income (loss):
Operating income	48.3	25.6	97.8	96.2
Depreciation and amortization:
Depreciation and amortization	10.8	9.4	21.1	18.8
Capital expenditures:
Capital expenditures	16.0	24.8	35.6	39.4
Assets:
Assets	1,097.6		1,097.6			1,046.8
All Other
Segment operating income (loss):
Operating income	(13.6)	(13.1)	(26.0)	(24.4)
Depreciation and amortization:
Depreciation and amortization	0.1	0.1	0.3	0.3
Capital expenditures:
Capital expenditures	0.1	$ 0.1	0.2	$ 0.3
Assets:
Assets	$ 326.2		$ 326.2			$ 338.4