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Pensions and Other Post-employment Benefit Obligations - Details of Net Defined Benefit Obligations by Country (Detail) - EUR (€)
€ in Millions
Jun. 30, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [line items]    
Defined benefit obligations € 1,030 € 1,051
Plan assets (387) (387)
Net defined benefit liability 643 664
France [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Defined benefit obligations 149 148
Plan assets (3) (3)
Net defined benefit liability 146 145
Germany [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Defined benefit obligations 139 142
Plan assets (1) (1)
Net defined benefit liability 138 141
Switzerland [Member]    
Disclosure of net defined benefit liability (asset) [line items]    
Defined benefit obligations 250 251
Plan assets (179) (177)
Net defined benefit liability 71 74
United States [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Defined benefit obligations 491 509
Plan assets (204) (206)
Net defined benefit liability 287 303
Other countries [member]    
Disclosure of net defined benefit liability (asset) [line items]    
Defined benefit obligations 1 1
Net defined benefit liability € 1 € 1