Segment and Geographical Area Information, Summary of financial information by operating segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Impairment of Intangible Assets						$ 2.6
Net Sales:
Net sales	$ 353.1	$ 332.8	$ 356.3	$ 355.3	$ 331.9	320.6	$ 334.9	$ 343.2	$ 1,397.5	$ 1,330.6	$ 1,391.9
Segment Operating Income (Loss):
Total operating income (loss)	40.0	$ 39.8	$ 11.4	$ 59.5	45.3	$ 29.8	$ 57.9	$ 44.8	150.7	177.8	(345.9)
Interest expense									(22.2)	(20.3)	(24.1)
Other income, net									4.5	(10.3)	(1.8)
Income (loss) before income taxes									133.0	147.2	(371.8)
Depreciation and Amortization:
Depreciation and amortization									39.7	36.0	32.4
Capital expenditures:
Capital expenditures									75.5	76.1	63.1
Segment assets:
Total	1,385.2				1,443.5				1,385.2	1,443.5	1,246.9
Fabricated Products
Net Sales:
Net sales									1,397.5	1,330.6	1,391.9
Segment Operating Income (Loss):
Total operating income (loss)									201.3	229.6	190.8
Depreciation and Amortization:
Depreciation and amortization									39.0	35.4	31.9
Capital expenditures:
Capital expenditures									75.3	75.6	62.4
Segment assets:
Total	1,046.8				969.4				1,046.8	969.4	904.7
All Other
Segment Operating Income (Loss):
Total operating income (loss)									(50.6)	(51.8)	(536.7)
Depreciation and Amortization:
Depreciation and amortization									0.7	0.6	0.5
Capital expenditures:
Capital expenditures									0.2	0.5	0.7
Segment assets:
Total	$ 338.4				$ 474.1				$ 338.4	$ 474.1	$ 342.2