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Defined Benefit Plans - Summary of Actuarial Assumptions Used to Determine Defined Benefit Obligations (Detail)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Aegon United Kingdom [member]    
Demographic actuarial assumptions    
Mortality UK mortality table UK mortality table
Financial actuarial assumptions    
Discount rate 2.08% 2.94%
Price inflation 3.08% 3.32%
Aegon Nederland N.V. [member]    
Financial actuarial assumptions    
Discount rate 0.94% 1.74%
Salary increase rate Curve 2019 Curve 2018
Indexation 59.2% of Curve 2019 57.75% of Curve 2018
United States [member]    
Demographic actuarial assumptions    
Mortality US mortality table US mortality table
Financial actuarial assumptions    
Salary increase rate 4.00% 3.85%
Health care trend rate 6.60% 7.00%
United States [member] | Top of range [member]    
Financial actuarial assumptions    
Discount rate 3.23% 4.22%
United States [member] | Bottom of range [member]    
Financial actuarial assumptions    
Discount rate 3.02% 4.05%