Fair Value Measurements, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Financial Assets:
Derivative Asset	$ 1.6	$ 85.7
Deferred compensation plan assets	7.3	$ 7.3
Investment Percentage in Industrial Sector		51.00%
Investment Percentage in Financial Sector		37.00%
Investment Percentage in Utilities Sector		12.00%
Financial Liabilities:
Derivative Liabilities	(16.2)	$ (96.8)
Fair Value, Measurements, Recurring
Financial Assets:
Cash and cash equivalents	72.5	177.7
Deferred compensation plan assets	7.3	7.3
Total Assets	173.2	1,171.1
Financial Liabilities:
Convertible Notes, including Bifurcated Conversion Feature		(263.3)
Total Liabilities	(223.5)	(604.6)
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | Fixed income investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	15.7	394.3
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | Mortgage backed securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		30.1
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | Corporate debt securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		75.4
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | Equity investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	23.8	191.3
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | United States Treasuries
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		39.5
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | Municipal debt securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		1.8
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | Cash and money market investments
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	1.9	19.3
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | Asset backed securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		8.1
Fair Value, Measurements, Recurring | VEBAs and Canadian Pension Plan | Diversified investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	20.4	26.6
Fair Value, Measurements, Recurring | Senior Notes
Financial Liabilities:
Senior Notes	(207.3)	(244.5)
Fair Value, Measurements, Recurring | Debt Securities
Financial Assets:
Short-term investments	30.0	114.0
Fair Value, Measurements, Recurring | Aluminum | Option contracts | Purchase
Financial Assets:
Derivative Asset	0.2
Fair Value, Measurements, Recurring | Aluminum | Swap contracts | Purchase
Financial Assets:
Derivative Asset	0.3
Financial Liabilities:
Derivative Liabilities	(8.9)	(4.2)
Fair Value, Measurements, Recurring | Aluminum | Swap contracts | Sale
Financial Assets:
Derivative Asset	0.2
Financial Liabilities:
Derivative Liabilities	(0.1)
Fair Value, Measurements, Recurring | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.9	1.0
Fair Value, Measurements, Recurring | Natural Gas | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	(6.7)	(6.2)
Fair Value, Measurements, Recurring | Electricity | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities		(1.7)
Fair Value, Measurements, Recurring | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Asset		84.7
Fair Value, Measurements, Recurring | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities		(84.7)
Fair Value, Measurements, Recurring | Level 1
Financial Assets:
Cash and cash equivalents	40.3	29.5
Deferred compensation plan assets	0.0	0.0
Total Assets	56.9	123.2
Financial Liabilities:
Convertible Notes, including Bifurcated Conversion Feature		(263.3)
Total Liabilities	(207.3)	(507.8)
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | Fixed income investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	0.0	54.0
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | Mortgage backed securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | Corporate debt securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | Equity investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | United States Treasuries
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | Municipal debt securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | Cash and money market investments
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	1.9	19.3
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | Asset backed securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 1 | VEBAs and Canadian Pension Plan | Diversified investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	14.7	20.4
Fair Value, Measurements, Recurring | Level 1 | Senior Notes
Financial Liabilities:
Senior Notes	(207.3)	(244.5)
Fair Value, Measurements, Recurring | Level 1 | Debt Securities
Financial Assets:
Short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Aluminum | Option contracts | Purchase
Financial Assets:
Derivative Asset	0.0
Fair Value, Measurements, Recurring | Level 1 | Aluminum | Swap contracts | Purchase
Financial Assets:
Derivative Asset	0.0
Financial Liabilities:
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Aluminum | Swap contracts | Sale
Financial Assets:
Derivative Asset	0.0
Financial Liabilities:
Derivative Liabilities	0.0
Fair Value, Measurements, Recurring | Level 1 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Natural Gas | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Level 1 | Electricity | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities		0.0
Fair Value, Measurements, Recurring | Level 1 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Asset		0.0
Fair Value, Measurements, Recurring | Level 1 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities		0.0
Fair Value, Measurements, Recurring | Level 2
Financial Assets:
Cash and cash equivalents	32.2	148.2
Short-term investments	30.0
Deferred compensation plan assets	7.3	7.3
Total Assets	115.4	1,046.9
Financial Liabilities:
Convertible Notes, including Bifurcated Conversion Feature		0.0
Total Liabilities	(15.9)	(96.8)
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | Fixed income investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	15.7	340.3
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | Mortgage backed securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		30.1
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | Corporate debt securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		75.4
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | Equity investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	23.8	191.3
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | United States Treasuries
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		39.5
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | Municipal debt securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		1.8
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | Cash and money market investments
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | Asset backed securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		8.1
Fair Value, Measurements, Recurring | Level 2 | VEBAs and Canadian Pension Plan | Diversified investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	5.7	6.2
Fair Value, Measurements, Recurring | Level 2 | Senior Notes
Financial Liabilities:
Senior Notes	0.0	0.0
Fair Value, Measurements, Recurring | Level 2 | Debt Securities
Financial Assets:
Short-term investments		114.0
Fair Value, Measurements, Recurring | Level 2 | Aluminum | Option contracts | Purchase
Financial Assets:
Derivative Asset	0.2
Fair Value, Measurements, Recurring | Level 2 | Aluminum | Swap contracts | Purchase
Financial Assets:
Derivative Asset	0.3
Financial Liabilities:
Derivative Liabilities	(8.9)	(4.2)
Fair Value, Measurements, Recurring | Level 2 | Aluminum | Swap contracts | Sale
Financial Assets:
Derivative Asset	0.2
Financial Liabilities:
Derivative Liabilities	(0.1)
Fair Value, Measurements, Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.0	0.0
Fair Value, Measurements, Recurring | Level 2 | Natural Gas | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	(6.7)	(6.2)
Fair Value, Measurements, Recurring | Level 2 | Electricity | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities		(1.7)
Fair Value, Measurements, Recurring | Level 2 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Asset		84.7
Fair Value, Measurements, Recurring | Level 2 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities		(84.7)
Fair Value, Measurements, Recurring | Level 3
Financial Assets:
Cash and cash equivalents	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total Assets	0.9	1.0
Financial Liabilities:
Convertible Notes, including Bifurcated Conversion Feature		0.0
Total Liabilities	(0.3)	0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | Fixed income investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | Mortgage backed securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | Corporate debt securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | Equity investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | United States Treasuries
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | Municipal debt securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | Cash and money market investments
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | Asset backed securities
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets		0.0
Fair Value, Measurements, Recurring | Level 3 | VEBAs and Canadian Pension Plan | Diversified investment funds in registered investment companies
Financial Assets:
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Senior Notes
Financial Liabilities:
Senior Notes	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Debt Securities
Financial Assets:
Short-term investments	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Aluminum | Option contracts | Purchase
Financial Assets:
Derivative Asset	0.0
Fair Value, Measurements, Recurring | Level 3 | Aluminum | Swap contracts | Purchase
Financial Assets:
Derivative Asset	0.0
Financial Liabilities:
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Aluminum | Swap contracts | Sale
Financial Assets:
Derivative Asset	0.0
Financial Liabilities:
Derivative Liabilities	0.0
Fair Value, Measurements, Recurring | Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset		1.0
Fair Value, Measurements, Recurring | Level 3 | Natural Gas | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Electricity | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities		0.0
Fair Value, Measurements, Recurring | Level 3 | Hedges Relating to the Convertible Notes | Call Options
Financial Assets:
Derivative Asset		0.0
Fair Value, Measurements, Recurring | Level 3 | Hedges Relating to the Convertible Notes | Bifurcated Conversion Feature
Financial Liabilities:
Derivative Liabilities		$ 0.0
Income Approach Valuation Technique | Fair Value, Measurements, Recurring | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liabilities	(0.3)
Income Approach Valuation Technique | Fair Value, Measurements, Recurring | Level 1 | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liabilities	0.0
Income Approach Valuation Technique | Fair Value, Measurements, Recurring | Level 2 | Aluminum | Midwest premium swap contracts
Financial Liabilities:
Derivative Liabilities	0.0
Income Approach Valuation Technique | Fair Value, Measurements, Recurring | Level 3 | Aluminum | Midwest premium swap contracts
Financial Assets:
Derivative Asset	0.9
Financial Liabilities:
Derivative Liabilities	(0.3)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Foreign Currency | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	(0.2)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 1 | Foreign Currency | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	0.0
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 2 | Foreign Currency | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	(0.2)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Level 3 | Foreign Currency | Swap contracts | Purchase
Financial Liabilities:
Derivative Liabilities	$ 0.0