Employee Incentive Plans, Compensation expense under LTI programs (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 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		$ 3.0	$ 2.4	$ 5.6	$ 4.6
Restricted Stock And Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		1.2	1.1	2.3	2.2
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		0.0	0.3	0.3	0.7
TSR-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		1.4	1.0	2.5	1.7
CP-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		0.4	0.0	0.5	0.0
Fabricated Products
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		1.1	0.9	1.9	1.6
All Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		$ 1.9	$ 1.5	$ 3.7	$ 3.0
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).