Segment Information - Summary of Results of Operations Measured as Income Before Federal Income Taxes and Equity in Earnings of Unconsolidated Affiliates by Operating Segments (Detail) - USD ($) $ 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
Segment Reporting Information [Line Items]
Premiums									$ 2,182,794	$ 2,228,193	$ 2,067,202
Other policy revenues									305,256	285,549	248,526
Net investment income									1,077,406	858,367	966,077
Net realized investment gains									30,751	16,931	91,209
Net gains (losses) on equity securities									422,535	(107,188)	0
Other income									51,401	44,530	37,986
Total premiums and other revenues	$ 1,057,273	$ 932,500	$ 956,677	$ 1,123,693	$ 518,700	$ 1,052,236	$ 952,071	$ 803,375	4,070,143	3,326,382	3,411,000
Policyholder benefits									667,828	708,313	681,122
Claims incurred									1,151,166	1,171,659	1,037,081
Interest credited to policyholders’ account balances									511,999	315,684	415,190
Commissions for acquiring and servicing policies									532,634	564,054	545,405
Other operating expenses									524,888	497,011	485,340
Deferred policy acquisition costs									(12,749)	(71,497)	(81,484)
Total benefits, losses and expenses	835,864	848,134	852,773	838,995	682,868	868,969	850,796	782,591	3,375,766	3,185,224	3,082,654
Income before federal income tax and other items	$ 221,409	$ 84,366	$ 103,904	$ 284,698	$ (164,168)	$ 183,267	$ 101,275	$ 20,784	694,377	141,158	328,346
Life
Segment Reporting Information [Line Items]
Premiums									359,419	350,012	328,570
Other policy revenues									288,061	270,839	234,979
Net investment income									263,788	233,181	245,835
Net realized investment gains									0	0	0
Net gains (losses) on equity securities									0	0
Other income									1,967	2,266	2,256
Total premiums and other revenues									913,235	856,298	811,640
Policyholder benefits									449,252	417,702	410,152
Claims incurred									0	0	0
Interest credited to policyholders’ account balances									80,950	54,249	73,965
Commissions for acquiring and servicing policies									162,203	158,657	147,176
Other operating expenses									190,104	190,835	190,482
Deferred policy acquisition costs									(26,036)	(33,893)	(49,786)
Total benefits, losses and expenses									856,473	787,550	771,989
Income before federal income tax and other items									56,762	68,748	39,651
Annuity
Segment Reporting Information [Line Items]
Premiums									147,139	231,027	222,207
Other policy revenues									17,195	14,710	13,547
Net investment income									663,895	467,788	573,789
Net realized investment gains									0	0	0
Net gains (losses) on equity securities									0	0
Other income									2,727	2,611	2,832
Total premiums and other revenues									830,956	716,136	812,375
Policyholder benefits									218,576	290,611	270,970
Claims incurred									0	0	0
Interest credited to policyholders’ account balances									431,049	261,435	341,225
Commissions for acquiring and servicing policies									71,350	94,879	105,389
Other operating expenses									50,507	46,859	44,486
Deferred policy acquisition costs									9,474	(35,135)	(30,022)
Total benefits, losses and expenses									780,956	658,649	732,048
Income before federal income tax and other items									50,000	57,487	80,327
Health
Segment Reporting Information [Line Items]
Premiums									165,035	180,414	156,436
Other policy revenues									0	0	0
Net investment income									9,467	9,376	9,538
Net realized investment gains									0	0	0
Net gains (losses) on equity securities									0	0
Other income									20,762	24,185	19,284
Total premiums and other revenues									195,264	213,975	185,258
Policyholder benefits									0	0	0
Claims incurred									109,013	122,547	103,037
Interest credited to policyholders’ account balances									0	0	0
Commissions for acquiring and servicing policies									31,624	32,516	27,400
Other operating expenses									41,475	41,819	38,475
Deferred policy acquisition costs									1,382	2,846	3,814
Total benefits, losses and expenses									183,494	199,728	172,726
Income before federal income tax and other items									11,770	14,247	12,532
Property & Casualty
Segment Reporting Information [Line Items]
Premiums									1,511,201	1,466,740	1,359,989
Other policy revenues									0	0	0
Net investment income									64,263	62,320	61,688
Net realized investment gains									0	0	0
Net gains (losses) on equity securities									0	0
Other income									11,897	10,628	8,372
Total premiums and other revenues									1,587,361	1,539,688	1,430,049
Policyholder benefits									0	0	0
Claims incurred									1,042,153	1,049,112	934,044
Interest credited to policyholders’ account balances									0	0	0
Commissions for acquiring and servicing policies									267,457	278,002	265,440
Other operating expenses									201,580	186,019	177,345
Deferred policy acquisition costs									2,431	(5,315)	(5,490)
Total benefits, losses and expenses									1,513,621	1,507,818	1,371,339
Income before federal income tax and other items									73,740	31,870	58,710
Corporate & Other
Segment Reporting Information [Line Items]
Premiums									0	0	0
Other policy revenues									0	0	0
Net investment income									75,993	85,702	75,227
Net realized investment gains									30,751	16,931	91,209
Net gains (losses) on equity securities									422,535	(107,188)
Other income									14,048	4,840	5,242
Total premiums and other revenues									543,327	285	171,678
Policyholder benefits									0	0	0
Claims incurred									0	0	0
Interest credited to policyholders’ account balances									0	0	0
Commissions for acquiring and servicing policies									0	0	0
Other operating expenses									41,222	31,479	34,552
Deferred policy acquisition costs									0	0	0
Total benefits, losses and expenses									41,222	31,479	34,552
Income before federal income tax and other items									$ 502,105	$ (31,194)	$ 137,126