Segment Information, Financial Information by Operating Segment Table (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Net sales:
Net Sales	$ 334.9	$ 367.2	$ 678.1	$ 738.9
Segment operating income (loss):
Operating income (loss)	57.9	37.0	102.7	(421.6)
Interest expense	(5.5)	(5.2)	(9.2)	(15.0)
Other income, net	(10.7)	0.4	(10.4)	0.8
Income (loss) before income taxes	41.7	32.2	83.1	(435.8)
Depreciation and amortization:
Depreciation and amortization	9.0	8.1	17.7	16.1
Capital expenditures:
Capital expenditures	16.4	11.6	42.3	22.9
Assets:
Assets	1,421.6		1,421.6		$ 1,247.6
Unrealized Gain (Loss) on Derivatives			14.9	(6.0)
Non-cash defined benefit net periodic benefit cost relating to Salaried VEBA	0.9	0.6	1.7	1.2
Loss (gain) on removal of Union VEBA net assets	0.0	1.6	(0.1)	493.8
Fabricated Products
Net sales:
Net Sales	334.9	367.2	678.1	738.9
Segment operating income (loss):
Operating income (loss)	72.1	50.6	130.0	95.5
Depreciation and amortization:
Depreciation and amortization	8.8	8.0	17.4	15.9
Capital expenditures:
Capital expenditures	16.3	11.5	42.1	22.7
Assets:
Assets	947.0		947.0		904.7
Unrealized Gain (Loss) on Derivatives	10.9	(1.5)	14.9	(6.0)
All Other
Segment operating income (loss):
Operating income (loss)	(14.2)	(13.6)	(27.3)	(517.1)
Depreciation and amortization:
Depreciation and amortization	0.2	0.1	0.3	0.2
Capital expenditures:
Capital expenditures	0.1	0.1	0.2	0.2
Assets:
Assets	474.6		474.6		$ 342.9
VEBAs
Assets:
Non-cash defined benefit net periodic benefit cost relating to Salaried VEBA	0.9	0.6	1.7	1.2
Loss (gain) on removal of Union VEBA net assets	$ 0.0	$ 1.6	$ (0.1)	$ 493.8