Selected Quarterly Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Premium income	$ 906,684	$ 899,993	$ 897,484	$ 890,973	$ 857,071	$ 860,750	$ 853,979	$ 850,106	$ 3,595,134	$ 3,421,906	$ 3,282,935
Net investment income	227,456	228,905	227,425	226,673	224,233	221,627	218,568	218,084	910,459	882,512	847,885
Realized gains (losses)	2,195	11,943	5,154	1,329	(16,600)	1,032	11,813	1,951	20,621	(1,804)	23,611
Total revenue	1,136,576	1,141,279	1,130,461	1,119,216	1,064,737	1,083,802	1,084,776	1,070,436	4,527,532	4,303,751	4,155,573
Policyholder benefits	594,538	585,692	589,362	587,757	569,120	567,856	568,377	569,889	2,357,349	2,275,242	2,227,875
Amortization of deferred policy acquisition costs	139,290	138,449	138,165	135,822	128,501	129,492	129,077	129,620	551,726	516,690	490,403
Pretax income from continuing operations	227,088	247,330	228,760	228,101	203,635	220,330	226,864	212,842	931,279	863,671	830,648
Income from continuing operations	187,061	201,818	186,609	185,394	164,706	178,700	184,393	173,711	760,882	701,510	1,458,263
Income (loss) from discontinued operations	0	0	(43)	(49)	11	24	32	(111)	(92)	(44)	(3,769)
Net income	$ 187,061	$ 201,818	$ 186,566	$ 185,345	$ 164,717	$ 178,724	$ 184,425	$ 173,600	$ 760,790	$ 701,466	$ 1,454,494
Basic net income (loss) per common share:
Continuing operations (in dollars per share)	$ 1.73	$ 1.85	$ 1.70	$ 1.68	$ 1.48	$ 1.59	$ 1.63	$ 1.52	$ 6.97	$ 6.22	$ 12.53
Discontinued operations (in dollars per share)	0	0	0	0	0	0	0	0	0	0	(0.03)
Total basic net income per common share (in dollars per share)	1.73	1.85	1.70	1.68	1.48	1.59	1.63	1.52	6.97	6.22	12.50
Diluted net income (loss) per common share:
Continuing operations (in dollars per share)	1.69	1.82	1.67	1.65	1.45	1.55	1.59	1.49	6.83	6.09	12.26
Discontinued operations (in dollars per share)	0	0	0	0	0	0	0	0	0	0	(0.04)
Total diluted net income per common share (in dollars per share)	$ 1.69	$ 1.82	$ 1.67	$ 1.65	$ 1.45	$ 1.55	$ 1.59	$ 1.49	$ 6.83	$ 6.09	$ 12.22