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Taxation - Additional Information (Detail) - EUR (€)
€ in Millions
 12 Months Ended
Dec. 22, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of changes in deferred tax liability (asset) [Abstract]        
Tax benefit € 578 € (578) € 0 € 0
Federal tax rate 21.00%      
Unused tax losses for which deferred tax asset recognised or not   4,676 4,138  
Unused tax credits for which deferred tax asset recognised or not   612 644  
Unused tax losses for which no deferred tax asset recognised   4,179 3,622  
Unused tax credits for which no deferred tax asset recognised   612 629  
Tax losses and credits for which deferred tax has not been recognized and which have a date to expire   2,934 2,363  
Other deductible temporary differences for which no deferred tax asset recognised   51 52  
Aggregate amount of temporary differences associated with undistributed earnings of subsidiaries for which deferred tax liabilities have not been recognised   € 1,719 € 1,557