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Defined Benefit Plans - Summary of Defined Benefit Plans (Detail) - EUR (€)
€ in Millions
Dec. 31, 2019
Dec. 31, 2018
Jan. 01, 2018
 [1]
Dec. 31, 2017
Disclosure of defined benefit plans [Line Items]        
Defined benefit plans € 4,359 € 3,989   € 3,950
Defined benefit assets 1   € 55  
Defined benefit liabilities 4,360 3,989 [1] € 4,005  
Retirement Benefit Plans [member]        
Disclosure of defined benefit plans [Line Items]        
Defined benefit plans 4,076 3,714   3,657
Other Post Employment Benefit Plans [member]        
Disclosure of defined benefit plans [Line Items]        
Defined benefit plans 283 275   € 293
Retirement benefit plans in surplus [member]        
Disclosure of defined benefit plans [Line Items]        
Defined benefit assets 1      
Retirement benefit plans in deficit [member]        
Disclosure of defined benefit plans [Line Items]        
Defined benefit liabilities 4,077 3,714    
Other post-employment benefit plans in deficit [member]        
Disclosure of defined benefit plans [Line Items]        
Defined benefit liabilities € 283 € 275    
[1] Amounts have been restated to reflect voluntary changes in accounting policies related to liability adequacy testing that was adopted by Aegon effective January 1, 2019. Refer to note 2.1.2 Voluntary changes in accounting policies for details about this change.