Summary of Significant Observable Inputs Used for Fair Value Measurements for Living Benefits Liabilities Classified as Level 3 (Parenthetical) (Detail) - Living Benefits [Member]	12 Months Ended
	Dec. 31, 2018 Person
Minimum [Member]
Summary of significant unobservable inputs used for fair value measurements for living benefits liabilities
Policyholders of mortality for majority of business	45
Maximum [Member]
Summary of significant unobservable inputs used for fair value measurements for living benefits liabilities
Policyholders of mortality for majority of business	85