Financial Instruments and Risk Management - General (Details) $ / shares in Units, € in Millions, $ in Millions, £ in Billions			1 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) item approach	Jun. 30, 2016 USD ($)	Jun. 30, 2016 EUR (€)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 EUR (€) item	Mar. 31, 2016 USD ($) item	Sep. 30, 2016 USD ($) item approach $ / shares	Sep. 30, 2017 USD ($)	Jun. 30, 2016 GBP (£)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 EUR (€)	Dec. 31, 2015 USD ($)
Financial Instruments and Risk Management
Period within which derivative will expire							5 years
Long-term debt	$ 7,803.0						$ 7,803.0					$ 5,028.0
Aggregate fair value of derivative instruments with credit-risk-related contingent features that were in a net liability position	$ 4.0						$ 4.0					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
4.375% Senior Notes, due December 2020
Financial Instruments and Risk Management
Long-term debt												$ 1,000.0
Interest rate (as a percent)	4.375%						4.375%				4.375%	4.375%
Senior Notes, due 2020 and 2023
Financial Instruments and Risk Management
Long-term debt | €											€ 1,100
3.50% Senior Notes, euro denominated, due December 2020
Financial Instruments and Risk Management
Long-term debt | €											€ 400
Interest rate (as a percent)	3.50%						3.50%				3.50%	3.50%
4.375% Senior Notes, euro denominated, due December 2023
Financial Instruments and Risk Management
Long-term debt | €											€ 700
Interest rate (as a percent)	4.375%						4.375%				4.375%	4.375%
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
Notional amount of derivatives	$ 708.0						$ 708.0
Impact on derivative contracts included in accumulated other comprehensive earnings (loss), net of tax	2.0						2.0
Commodity contracts | Derivatives Designated As Hedging Instruments | Cash Flow Hedging
Financial Instruments and Risk Management
Notional amount of derivatives	681.0						$ 681.0
Commodity contracts | Forecast
Financial Instruments and Risk Management
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months								$ 2.0
Commodity contracts | Maximum
Financial Instruments and Risk Management
Period within which derivative will expire							3 years
Interest rate swap agreements
Financial Instruments and Risk Management
Notional amount of derivatives	750.0						$ 750.0
Impact on derivative contracts included in accumulated other comprehensive earnings (loss), net of tax	(1.0)						$ (1.0)
Interest rate swap agreements | Derivatives Designated As Hedging Instruments
Financial Instruments and Risk Management
Number Of Contracts Designated As Hedges | item							0
Interest rate swap agreements | Rexam | Derivatives Not Designated As Hedging Instruments
Financial Instruments and Risk Management
Notional amount terminated				$ 200.0
Interest rate swap agreements | Rexam | Derivatives Designated As Hedging Instruments
Financial Instruments and Risk Management
Number Of Contracts Designated As Hedges | item					0	0
Interest rate swap agreements | Rexam | Derivatives Not Designated As Hedging Instruments
Financial Instruments and Risk Management
Notional amount terminated					€ 850	$ 923.0
Interest rate swaption | Rexam | Derivatives Not Designated As Hedging Instruments
Financial Instruments and Risk Management
Notional amount terminated | €			€ 1,600
Currency Exchange Rate Risk | Rexam
Financial Instruments and Risk Management
Notional amount of derivatives	1,800.0						$ 1,800.0		£ 1.4
Currency Exchange Rate Risk | Registrant Excluding Effects Of Acquisition [Member]
Financial Instruments and Risk Management
Notional amount of derivatives	1,900.0						$ 1,900.0
Period within which derivative will expire							2 years
Net after-tax gain (loss) included in AOCI	(3.0)
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months								3.0
Cross-currency swap
Financial Instruments and Risk Management
Notional amount terminated		$ 1,000.0
Amount of hedge contract	1,000.0						$ 1,000.0
Impact on derivative contracts included in accumulated other comprehensive earnings (loss), net of tax	$ 9.0						$ 9.0
Cross-currency swap | Rexam | Derivatives Not Designated As Hedging Instruments
Financial Instruments and Risk Management
Notional amount terminated		$ 705.0
Cross-currency swap | Forecast
Financial Instruments and Risk Management
Net gain (loss) expected to be recognized in the consolidated statement of earnings during the next 12 months								$ 0.0
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						1,300,000
Equity contracts | Maximum
Financial Instruments and Risk Management
Impact of change in the company's stock price on pretax earnings							$ 1.0