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Pensions and Other Post-employment Benefit Obligations - Additional Information (Detail) - EUR (€)
€ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of defined benefit plans [line items]        
Gain from negative past service cost € 36      
Weighted-average maturity of defined benefit obligations     13 years 3 months 18 days 14 years
Pensions [member]        
Disclosure of defined benefit plans [line items]        
Expected contributions to pension and other benefits   € 26    
Other benefits [member]        
Disclosure of defined benefit plans [line items]        
Expected contributions to pension and other benefits   € 19    
Actuarial assumption of discount rates [member]        
Disclosure of defined benefit plans [line items]        
Increase in actuarial assumption     0.50%  
Decrease in actuarial assumption     0.50%