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Defined Benefit Plans - Summary of Target Allocation of Plan Assets for Retirement Benefit Plans for the Next Annual Period (Detail)
Dec. 31, 2019
Dec. 31, 2018
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Equity instruments 4.00% 3.00%
Debt instruments 19.00% 20.00%
Aegon United Kingdom [member] | Actuarial assumption of expected rates of pension increases [member] | Defined benefit plans [member]    
Disclosure of sensitivity analysis for actuarial assumptions [Line Items]    
Equity instruments   32.00%
Debt instruments   68.00%