Investments - Schedule of Analysis of Investment Operations (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net Investment Income [Line Items]
Investment income, gross	$ 233,175	$ 230,606
Less investment expense	(4,184)	(3,933)
Net investment income	$ 228,991	226,673
Percentage change in investment income, gross	1.00%
Percentage change in investment expense	6.00%
Percentage change in net investment income	1.00%
Fixed maturities available for sale
Net Investment Income [Line Items]
Investment income, gross	$ 217,127	215,763
Percentage change in investment income, gross	1.00%
Policy loans
Net Investment Income [Line Items]
Investment income, gross	$ 11,118	10,636
Percentage change in investment income, gross	5.00%
Other long-term investments
Net Investment Income [Line Items]
Investment income, gross	$ 4,623	3,388
Percentage change in investment income, gross	36.00%
Short-term investments
Net Investment Income [Line Items]
Investment income, gross	$ 307	819
Percentage change in investment income, gross	(63.00%)
Fair value option
Net Investment Income [Line Items]
Net investment income	$ 2,000	$ 1,300