Segment Information - Summary of Results of Operations Measured as Income Before Federal Income Taxes and Other Items by Operating Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Premiums	$ 556,272	$ 560,740	$ 1,092,509	$ 1,105,720
Other policy revenues	75,146	71,138	149,394	142,477
Net investment income	257,730	246,741	550,076	455,410
Net realized investment losses	(9,905)	5,007	(6,958)	6,051
Net gains on equity securities	67,060	57,162	273,437	23,992
Other income	10,374	11,283	21,912	21,796
Total premiums and other revenues	956,677	952,071	2,080,370	1,755,446
Policyholder benefits	177,250	179,061	345,476	362,353
Claims incurred	298,316	312,436	562,227	583,066
Interest credited to policyholders’ account balances	123,687	105,731	264,921	176,276
Commissions for acquiring and servicing policies	141,295	149,737	279,940	294,433
Other operating expenses	129,533	123,947	263,143	254,341
Change in deferred policy acquisition costs	(17,308)	(20,116)	(23,939)	(37,082)
Total benefits, losses and expenses	852,773	850,796	1,691,768	1,633,387
Income before federal income tax and other items	103,904	101,275	388,602	122,059
Life
Segment Reporting Information [Line Items]
Premiums	86,087	84,595	172,555	165,971
Other policy revenues	70,610	67,231	140,854	134,962
Net investment income	63,916	61,082	132,672	118,850
Net realized investment losses	0	0	0	0
Net gains on equity securities	0	0	0	0
Other income	469	512	1,145	1,267
Total premiums and other revenues	221,082	213,420	447,226	421,050
Policyholder benefits	104,462	96,958	213,927	195,504
Claims incurred	0	0	0	0
Interest credited to policyholders’ account balances	15,197	21,046	35,516	37,311
Commissions for acquiring and servicing policies	40,881	39,391	78,623	78,911
Other operating expenses	46,669	48,189	95,647	99,139
Change in deferred policy acquisition costs	(9,205)	(7,249)	(14,040)	(13,692)
Total benefits, losses and expenses	198,004	198,335	409,673	397,173
Income before federal income tax and other items	23,078	15,085	37,553	23,877
Annuity
Segment Reporting Information [Line Items]
Premiums	56,222	67,228	96,129	137,844
Other policy revenues	4,536	3,907	8,540	7,515
Net investment income	155,355	148,710	346,066	262,190
Net realized investment losses	0	0	0	0
Net gains on equity securities	0	0	0	0
Other income	574	631	1,263	1,356
Total premiums and other revenues	216,687	220,476	451,998	408,905
Policyholder benefits	72,788	82,103	131,549	166,849
Claims incurred	0	0	0	0
Interest credited to policyholders’ account balances	108,490	84,685	229,405	138,965
Commissions for acquiring and servicing policies	23,139	30,355	50,005	60,359
Other operating expenses	13,349	11,853	25,823	23,172
Change in deferred policy acquisition costs	(4,369)	(8,811)	(7,389)	(17,684)
Total benefits, losses and expenses	213,397	200,185	429,393	371,661
Income before federal income tax and other items	3,290	20,291	22,605	37,244
Health
Segment Reporting Information [Line Items]
Premiums	42,224	48,870	80,905	89,885
Other policy revenues	0	0	0	0
Net investment income	2,376	2,263	4,796	4,617
Net realized investment losses	0	0	0	0
Net gains on equity securities	0	0	0	0
Other income	5,534	6,809	10,919	11,966
Total premiums and other revenues	50,134	57,942	96,620	106,468
Policyholder benefits	0	0	0	0
Claims incurred	26,708	32,310	52,475	60,450
Interest credited to policyholders’ account balances	0	0	0	0
Commissions for acquiring and servicing policies	8,692	9,126	15,570	15,142
Other operating expenses	10,707	10,090	21,699	20,448
Change in deferred policy acquisition costs	(146)	506	661	1,594
Total benefits, losses and expenses	45,961	52,032	90,405	97,634
Income before federal income tax and other items	4,173	5,910	6,215	8,834
Property & Casualty
Segment Reporting Information [Line Items]
Premiums	371,739	360,047	742,920	712,020
Other policy revenues	0	0	0	0
Net investment income	16,706	15,493	31,728	31,354
Net realized investment losses	0	0	0	0
Net gains on equity securities	0	0	0	0
Other income	2,830	2,264	5,552	4,327
Total premiums and other revenues	391,275	377,804	780,200	747,701
Policyholder benefits	0	0	0	0
Claims incurred	271,608	280,126	509,752	522,616
Interest credited to policyholders’ account balances	0	0	0	0
Commissions for acquiring and servicing policies	68,583	70,865	135,742	140,021
Other operating expenses	49,681	45,166	101,566	92,967
Change in deferred policy acquisition costs	(3,588)	(4,562)	(3,171)	(7,300)
Total benefits, losses and expenses	386,284	391,595	743,889	748,304
Income before federal income tax and other items	4,991	(13,791)	36,311	(603)
Corporate & Other
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Other policy revenues	0	0	0	0
Net investment income	19,377	19,193	34,814	38,399
Net realized investment losses	(9,905)	5,007	(6,958)	6,051
Net gains on equity securities	67,060	57,162	273,437	23,992
Other income	967	1,067	3,033	2,880
Total premiums and other revenues	77,499	82,429	304,326	71,322
Policyholder benefits	0	0	0	0
Claims incurred	0	0	0	0
Interest credited to policyholders’ account balances	0	0	0	0
Commissions for acquiring and servicing policies	0	0	0	0
Other operating expenses	9,127	8,649	18,408	18,615
Change in deferred policy acquisition costs	0	0	0	0
Total benefits, losses and expenses	9,127	8,649	18,408	18,615
Income before federal income tax and other items	$ 68,372	$ 73,780	$ 285,918	$ 52,707