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Financial Risk Management - Summary of Effect of Foreign Currency Derivatives Impacts and Commercial Transactions Exposures (Detail) - EUR (€)
€ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Realized gain on foreign currency derivatives-net € (15) € (46) € (37)
Unrealized gain on foreign currency derivatives-net 17 40 (10)
Realized gain/(loss) in ineffective portion of derivatives 0 0 0
Derivatives that do not qualify for hedge accounting [member] | Other gains and losses [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Realized gain on foreign currency derivatives-net (19) (46) (37)
Unrealized gain on foreign currency derivatives-net 16 40 € (10)
Derivatives that qualify for hedge accounting [member] | Other gains and losses [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Realized gain on foreign currency derivatives-net 3    
Derivatives that qualify for hedge accounting [member] | Revenue [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Realized gain on foreign currency derivatives-net 1    
Unrealized gain on foreign currency derivatives-net 1    
Derivatives that qualify for hedge accounting [member] | Other comprehensive income (loss) [member]      
Disclosure of nature and extent of risks arising from financial instruments [line items]      
Unrealized gain on foreign currency derivatives-net 48 € (27)  
Gain/loss reclassified from cash flow hedge reserve to profit and loss € (2)