Investments - Schedule of Analysis of Investment Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Investment Income [Line Items]
Investment income, gross	$ 236,049	$ 231,442	$ 469,224	$ 462,048
Less investment expense	(4,481)	(4,017)	(8,665)	(7,950)
Net investment income	$ 231,568	227,425	$ 460,559	454,098
Percentage change in investment income, gross	2.00%		2.00%
Percentage change in investment expense	12.00%		9.00%
Percentage change in net investment income	2.00%		1.00%
Fixed maturities available for sale
Net Investment Income [Line Items]
Investment income, gross	$ 217,556	215,867	$ 434,683	431,630
Percentage change in investment income, gross	1.00%		1.00%
Policy loans
Net Investment Income [Line Items]
Investment income, gross	$ 11,213	10,821	$ 22,331	21,457
Percentage change in investment income, gross	4.00%		4.00%
Other long-term investments
Net Investment Income [Line Items]
Investment income, gross	$ 7,146	3,991	$ 11,769	7,379
Percentage change in investment income, gross	79.00%		59.00%
Short-term investments
Net Investment Income [Line Items]
Investment income, gross	$ 134	763	$ 441	1,582
Percentage change in investment income, gross	(82.00%)		(72.00%)
Fair value option
Net Investment Income [Line Items]
Net investment income	$ 4,500	$ 1,400	$ 6,500	$ 2,800