Financial Instruments and Risk Management - General (Details) $ / shares in Units, € in Millions, £ in Millions, $ in Millions	1 Months Ended		12 Months Ended
	Jun. 30, 2016 GBP (£)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) item $ / shares	Dec. 31, 2015 USD ($)	Dec. 31, 2015 EUR (€)	Dec. 31, 2015 GBP (£)	Dec. 31, 2015 USD ($)
Financial Instruments and Risk Management
Notional amount terminated | £	£ 1,400
Costs associated with terminated interest rate contracts			$ 20.0	$ 16.0
Long-term debt			7,389.0				$ 5,028.0
Long-term Debt, Gross			7,400.0				5,100.0
Aggregate fair value of derivative instruments with credit-risk-related contingent features that were in a net liability position			44.0				69.0
Collateral amount posted for derivative instruments with credit-risk-related contingent features that were in a net liability position			$ 0.0				0.0
Cash Flow Hedging
Financial Instruments and Risk Management
Period within which derivative will expire			3 years
Senior Notes 4.375 Percent Due December 2020
Financial Instruments and Risk Management
Long-term debt							1,000.0
Long-term Debt, Gross			$ 1,000.0				$ 1,000.0
Interest rate (as a percent)			4.375%		4.375%	4.375%	4.375%
Senior Notes 4.375 Percent Due December 2023
Financial Instruments and Risk Management
Long-term debt | €					€ 700
Long-term Debt, Gross			$ 736.0				$ 760.0
Interest rate (as a percent)			4.375%		4.375%	4.375%	4.375%
Senior Notes 3.50 Percent Due December 2020
Financial Instruments and Risk Management
Long-term debt | €					€ 400
Long-term Debt, Gross			$ 421.0				$ 435.0
Interest rate (as a percent)			3.50%		3.50%	3.50%	3.50%
Intercompany Debt Foreign Currency Denominated
Financial Instruments and Risk Management
Gain on derivatives included in accumulated other comprehensive earnings (loss), net of tax			$ 37.0
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months			$ 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 | item			2
Notional amount of derivatives			$ 705.0
Gain on derivatives included in accumulated other comprehensive earnings (loss), net of tax			17.0
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months			12.0
Commodity contracts | Derivatives Designated As Hedging Instruments | Cash Flow Hedging
Financial Instruments and Risk Management
Notional amount of derivatives			643.0
Interest rate swap agreements
Financial Instruments and Risk Management
Notional amount of derivatives			$ 500.0
Period within which derivative will expire			1 year
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months			$ 1.0
Currency Exchange Rate Risk
Financial Instruments and Risk Management
Notional amount of derivatives			$ 1.6
Period within which derivative will expire			2 years
Gain on derivatives included in accumulated other comprehensive earnings (loss), net of tax			$ (3.0)
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months			$ 4.0
Cross-currency swap
Financial Instruments and Risk Management
Notional amount of derivatives		$ 1,000.0
Period within which derivative will expire			4 years
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months			$ 39.0
Notional amount terminated		$ 1,000.0
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			1,300,000
Fair value of the swaps			$ (2.0)	$ 3.0
Equity contracts | Maximum
Financial Instruments and Risk Management
Impact of change in the company's stock price on pretax earnings			$ 1.0
U.S. Dollar | Rexam
Financial Instruments and Risk Management
Invested in escrow account							$ 987.0
British pound | Rexam
Financial Instruments and Risk Management
Invested in escrow account						£ 792	$ 1,167.0