Financial Instruments and Risk Management - General (Details) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) item approach	Jun. 30, 2017 USD ($) item approach $ / shares	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) item approach	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Financial Instruments and Risk Management
Costs associated with terminated interest rate contracts			$ 4.0		$ 20.0
Aggregate fair value of derivative instruments with credit-risk-related contingent features that were in a net liability position	$ 24.0	$ 24.0		$ 24.0		$ 44.0
Collateral amount posted for derivative instruments with credit-risk-related contingent features that were in a net liability position	$ 0.0	$ 0.0		$ 0.0		$ 0.0
Commodity contracts
Financial Instruments and Risk Management
Number of methods through which entity manages commodity price risk in connection with market price fluctuations of aluminum ingot | approach	2	2		2
Notional amount of derivatives	$ 985.0	$ 985.0		$ 985.0
Gain (loss) on derivatives included in accumulated other comprehensive earnings (loss), net of tax	23.0	23.0		23.0
Net gain expected to be recognized in the consolidated statement of earnings during the next 12 months				17.0
Commodity contracts | Derivatives Designated As Hedging Instruments | Cash Flow Hedging
Financial Instruments and Risk Management
Notional amount of derivatives	$ 922.0	$ 922.0		922.0
Commodity contracts | Maximum
Financial Instruments and Risk Management
Period within which derivative will expire		3 years
Interest rate swap agreements | Cash Flow Hedging
Financial Instruments and Risk Management
Net after-tax gain (loss) included in AOCI				$ 0.0
Number of contracts outstanding | item	0	0		0
Currency Exchange Rate Risk
Financial Instruments and Risk Management
Notional amount of derivatives	$ 2,000.0	$ 2,000.0		$ 2,000.0
Period within which derivative will expire				2 years
Net gain expected to be recognized in the consolidated statement of earnings during the next 12 months				$ 5.0
Net after-tax gain (loss) included in AOCI	5.0
Cross-currency swap
Financial Instruments and Risk Management
Gain (loss) on derivatives included in accumulated other comprehensive earnings (loss), net of tax	(13.0)	(13.0)		(13.0)
Amount of hedge contract			$ 1,000.0		$ 1,000.0
Fair value of swaps	$ (39.0)	$ (39.0)		$ (39.0)
Equity contracts
Financial Instruments and Risk Management
Change in company's stock price (in dollars per share) | $ / shares		$ 1
Notional value of the swap (in shares) | item	2,700,000	2,700,000		2,700,000
Fair value of swaps	$ 3.4	$ 3.4		$ 3.4