Selected Quarterly Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Quarterly Financial Information Disclosure [Abstract]
Total assets	$ 28,051,499	$ 26,223,345	$ 26,769,793	$ 26,922,329	$ 25,986,797	$ 25,248,899	$ 26,424,294	$ 28,215,723	$ 28,051,499	$ 25,986,797
Total liabilities	23,564,696	21,600,214	22,789,487	23,076,038	22,037,220	21,638,930	23,203,991	25,671,448	23,564,696	22,037,220
Premium income	1,130,672	1,119,335	1,110,920	1,095,090	1,088,949	1,079,282	1,077,199	1,064,812	4,456,017	4,310,242	$ 4,094,813
Net investment income	271,609	266,926	261,244	257,105	255,483	246,711	244,712	244,894	1,056,884	991,800	956,690
Realized gain (loss)—investments	13,287	(2,193)	(45,843)	(30,927)	(9,703)	(29,155)	(30,446)	(7,244)	(65,676)	(76,548)	59,319
Total revenue	1,415,691	1,384,118	1,326,406	1,321,318	1,335,113	1,297,237	1,291,764	1,302,626	5,447,533	5,226,740	5,112,038
Policyholder benefits	719,770	719,044	717,510	707,927	702,278	737,576	691,431	694,149	2,864,251	2,825,434	2,659,046
Amortization of deferred acquisition costs	97,541	95,757	94,080	92,322	90,131	88,012	86,185	84,496	379,700	348,824	317,616
Pretax income	336,711	318,815	264,506	274,234	296,710	234,776	276,449	294,176
Net income	$ 274,802	$ 257,083	$ 215,260	$ 223,610	$ 242,343	$ 190,586	$ 223,973	$ 237,484	$ 970,755	$ 894,386	$ 1,031,114
Basic net income per common share (in dollars per share)	$ 2.92	$ 2.72	$ 2.26	$ 2.32	$ 2.50	$ 1.96	$ 2.28	$ 2.39	$ 10.21	$ 9.13	$ 10.10
Diluted net income per common share (in dollars per share)	$ 2.88	$ 2.68	$ 2.24	$ 2.28	$ 2.46	$ 1.94	$ 2.26	$ 2.37	$ 10.07	$ 9.04	$ 9.99