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Defined Benefit Plans - Summary of Amounts Recognized in Statement of Financial Positions (Detail) - EUR (€)
€ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [Line Items]      
Present value of wholly or partly funded obligations € 4,807 € 4,027  
Fair value of plan assets (4,420) (3,525)  
Surplus Deficit In Plan 386 502  
Present value of wholly unfunded obligations 1) 3,973 3,487  
Ending Balance 4,359 3,989 € 3,950
Retirement Benefit Plans [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Present value of wholly or partly funded obligations 4,807 4,027  
Fair value of plan assets (4,420) (3,525)  
Surplus Deficit In Plan 386 502  
Present value of wholly unfunded obligations 1) 3,690 3,212  
Ending Balance 4,076 3,714 3,657
Other Post Employment Benefit Plans [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Present value of wholly unfunded obligations 1) 283 275  
Ending Balance € 283 € 275 € 293