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Pensions and Other Post-employment Benefit Obligations - Details of Net Defined Benefit Obligations by Country (Detail) - EUR (€)
€ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligations € 1,051 € 1,126  
Plan assets (387) (391)  
Net defined benefit liability 664 735 € 701
France [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligations 148 144  
Plan assets (3)    
Net defined benefit liability 145 144  
Germany [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligations 142 147  
Plan assets (1) (1)  
Net defined benefit liability 141 146  
Switzerland [Member]      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligations 251 284  
Plan assets (177) (181)  
Net defined benefit liability 74 103  
United States [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligations 509 550  
Plan assets (206) (209)  
Net defined benefit liability 303 341  
Other countries [member]      
Disclosure of net defined benefit liability (asset) [line items]      
Defined benefit obligations 1 1  
Net defined benefit liability € 1 € 1