Employee Incentive Plans, Compensation expense under LTI programs (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative-effect adjustment	[1]						$ 0.7
Allocated Share-based Compensation Expense		$ 3.1	$ 2.2	$ 8.7	$ 6.8
Non-vested common shares and restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		1.2	1.1	3.5	3.3
EVA-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		0.0	0.1	0.3	0.8
TSR-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		1.5	1.0	4.0	2.7
CP-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		0.4	0.0	0.9	0.0
Fabricated Products
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		1.2	0.8	3.1	2.4
All Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		$ 1.9	$ 1.4	$ 5.6	$ 4.4
Minimum | TSR-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target number of underlying shares (percent)		0.00%
Minimum | CP-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target number of underlying shares (percent)		0.00%
Maximum | TSR-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target number of underlying shares (percent)		200.00%
Maximum | CP-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of target number of underlying shares (percent)		200.00%
Additional Paid in Capital
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative-effect adjustment	[1]						0.8
Accounting Standards Update 2016-09 | New Accounting Pronouncement, Early Adoption, Effect
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative-effect adjustment	[1]					$ 0.7
Accounting Standards Update 2016-09 | New Accounting Pronouncement, Early Adoption, Effect | Additional Paid in Capital
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative-effect adjustment	[1]						$ 0.8

[1] See Note 4 and Note 6 for discussion of our adoption of ASU 2016-09 (as defined in Note 1).