Segment and Geographical Area Information, Summary of financial information by operating segment (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information
Impairment of Intangible Assets			$ 2.6
Net Sales:
Net sales		$ 331.9	320.6	$ 334.9	$ 343.2	$ 316.6	$ 336.4	$ 367.2	$ 371.7	$ 1,330.6	$ 1,391.9	$ 1,356.1
Segment Operating Income (Loss):
Total operating income (loss)		45.3	29.8	57.9	44.8	35.2	40.5	37.0	(458.6)	177.8	(345.9)	137.9
Interest expense										(20.3)	(24.1)	(37.5)
Other income, net										(10.3)	(1.8)	6.7
Income (loss) before income taxes										147.2	(371.8)	107.1
Depreciation and Amortization:
Depreciation and amortization										36.0	32.4	31.1
Capital expenditures:
Capital expenditures										76.1	63.1	59.4
Segment assets:
Total		1,443.5				1,246.9				1,443.5	1,246.9
Non-cash impairment charges										2.8	0.1	1.5
Lower of cost or market inventory write-down		0.0	$ 0.0	$ 0.0	$ 4.9	2.6	$ 0.0	$ 0.0	$ 0.0	4.9	2.6	0.0
Unrealized Gain (Loss) on Derivatives										18.7	(3.4)	(6.8)
Net periodic postretirement benefit cost (income) relating to VEBAs										3.4	2.4	(23.7)
Non-cash defined benefit net periodic benefit cost (income)	[1]									3.7	2.8	(23.5)
(Gain) loss on removal of Union VEBA net assets										(0.1)	493.4	0.0
Fabricated Products
Net Sales:
Net sales										1,330.6	1,391.9	1,356.1
Segment Operating Income (Loss):
Total operating income (loss)										229.6	190.8	151.4
Depreciation and Amortization:
Depreciation and amortization										35.4	31.9	30.6
Capital expenditures:
Capital expenditures										75.6	62.4	58.5
Segment assets:
Total		969.4				904.7				969.4	904.7
Unrealized Gain (Loss) on Derivatives										18.7	(3.4)	(10.4)
All Other
Segment Operating Income (Loss):
Total operating income (loss)										(51.8)	(536.7)	(13.5)
Depreciation and Amortization:
Depreciation and amortization										0.6	0.5	0.5
Capital expenditures:
Capital expenditures										0.5	0.7	0.9
Segment assets:
Total		$ 474.1				$ 342.2				474.1	342.2
VEBAs | All Other
Segment assets:
Non-cash defined benefit net periodic benefit cost (income)										$ 3.4	$ 2.4	$ (23.7)

[1] See Note 6 for the impact of removing the Union VEBA (defined in Note 6) net assets.