Investments - Summary of Investment Portfolio (Detail) - Credit Concentration Risk	12 Months Ended
	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	100.00%
Other
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	1.00%
Fixed maturities available for sale | Corporates | Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	82.00%
Fixed maturities available for sale | Corporates | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	3.00%
Fixed maturities available for sale | Securities of state and municipal governments | Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	7.00%
Fixed maturities available for sale | Government-sponsored enterprises | Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	2.00%
Fixed maturities available for sale | Other | Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	1.00%
Fixed maturities available for sale | Other | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	1.00%
Policy loans
Schedule of Available-for-sale Securities [Line Items]
Concentration risk percentage	3.00%