Investments - Schedule of Analysis of Investment Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Investment Income [Line Items]
Investment income, gross	$ 236,154	$ 232,732	$ 705,378	$ 694,780
Less investment expense	(4,722)	(3,827)	(13,387)	(11,777)
Net investment income	$ 231,432	228,905	$ 691,991	683,003
Percentage change in investment income, gross	1.00%		2.00%
Percentage change in investment expense	23.00%		14.00%
Percentage change in net investment income	1.00%		1.00%
Fixed maturities available for sale
Net Investment Income [Line Items]
Investment income, gross	$ 217,900	216,154	$ 652,583	647,784
Percentage change in investment income, gross	1.00%		1.00%
Policy loans
Net Investment Income [Line Items]
Investment income, gross	$ 11,241	10,959	$ 33,572	32,416
Percentage change in investment income, gross	3.00%		4.00%
Other long-term investments
Net Investment Income [Line Items]
Investment income, gross	$ 6,925	5,105	$ 18,694	12,484
Percentage change in investment income, gross	36.00%		50.00%
Short-term investments
Net Investment Income [Line Items]
Investment income, gross	$ 88	514	$ 529	2,096
Percentage change in investment income, gross	(83.00%)		(75.00%)
Fair value option
Net Investment Income [Line Items]
Net investment income	$ 4,000	$ 1,500	$ 10,500	$ 4,200