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Defined Benefit Plans - Summary of Fair Value of Plan Assets (Detail) - EUR (€)
€ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [Line Items]      
Current year service cost € 157 € 131 € 158
Net interest on the net defined benefit liability (asset) 89 96 109
Past service cost (1) (21) (6)
Total defined benefit expenses 244 206  
Retirement Benefit Plans [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current year service cost 148 121 148
Net interest on the net defined benefit liability (asset) 80 87 99
Past service cost (1) (16) (5)
Total defined benefit expenses 227 192 242
Other Post Employment Benefit Plans [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current year service cost 8 10 11
Net interest on the net defined benefit liability (asset) 9 8 10
Past service cost   (5) (1)
Total defined benefit expenses € 17 € 13 € 19