Segment Information, Financial Information by Operating Segment Table (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Net sales:
Net Sales	$ 320.6	$ 336.4	$ 998.7	$ 1,075.3
Segment operating income (loss):
Operating income (loss)	29.8	40.5	132.5	(381.1)
Interest expense	(5.5)	(4.9)	(14.7)	(19.9)
Other expense, net	0.0	(0.9)	(10.4)	(0.1)
Income (loss) before income taxes	24.3	34.7	107.4	(401.1)
Depreciation and amortization:
Depreciation and amortization	9.0	8.1	26.7	24.2
Capital expenditures:
Capital expenditures	15.1	15.5	57.4	38.4
Assets:
Assets	1,433.4		1,433.4		$ 1,247.6
Unrealized Gain (Loss) on Derivatives			16.9	(7.7)
Non-cash net periodic postretirement benefit cost relating to Salaried VEBA	0.8	0.6	2.5	1.8
(Gain) loss on removal of Union VEBA net assets	0.0	(2.9)	(0.1)	490.9
Fabricated Products
Net sales:
Net Sales	320.6	336.4	998.7	1,075.3
Segment operating income (loss):
Operating income (loss)	42.1	47.3	172.1	142.8
Depreciation and amortization:
Depreciation and amortization	8.8	7.9	26.2	23.8
Capital expenditures:
Capital expenditures	15.0	15.1	57.1	37.8
Assets:
Assets	962.3		962.3		904.7
Unrealized Gain (Loss) on Derivatives	2.0	(1.7)	16.9	(7.7)
All Other
Segment operating income (loss):
Operating income (loss)	(12.3)	(6.8)	(39.6)	(523.9)
Depreciation and amortization:
Depreciation and amortization	0.2	0.2	0.5	0.4
Capital expenditures:
Capital expenditures	0.1	0.4	0.3	0.6
Assets:
Assets	471.1		471.1		$ 342.9
VEBAs
Assets:
Non-cash net periodic postretirement benefit cost relating to Salaried VEBA	0.8	0.6	2.5	1.8
(Gain) loss on removal of Union VEBA net assets	$ 0.0	$ (2.9)	$ (0.1)	$ 490.9