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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Premiums	$ 557,844	$ 558,468	$ 1,650,353	$ 1,664,188
Other policy revenues	76,784	70,840	226,178	213,317
Net investment income	246,620	285,532	796,696	740,942
Net realized investment gains	31,933	(1,276)	24,975	4,775
Net gains on equity securities	8,589	126,495	282,026	150,487
Other income	10,730	12,177	32,642	33,973
Total premiums and other revenues	932,500	1,052,236	3,012,870	2,807,682
Policyholder benefits	173,351	183,969	518,827	546,322
Claims incurred	309,262	302,636	871,489	885,702
Interest credited to policyholders’ account balances	106,782	133,418	371,703	309,694
Commissions for acquiring and servicing policies	128,689	138,979	408,629	433,412
Other operating expenses	128,502	118,761	391,645	373,102
Change in deferred policy acquisition costs	1,548	(8,794)	(22,391)	(45,876)
Total benefits, losses and expenses	848,134	868,969	2,539,902	2,502,356
Income before federal income tax and other items	84,366	183,267	472,968	305,326
Life
Segment Reporting Information [Line Items]
Premiums	93,079	91,176	265,634	257,147
Other policy revenues	72,446	67,260	213,300	202,222
Net investment income	61,961	65,875	194,633	184,725
Net realized investment gains	0	0	0	0
Net gains on equity securities	0	0	0	0
Other income	441	492	1,586	1,759
Total premiums and other revenues	227,927	224,803	675,153	645,853
Policyholder benefits	113,652	119,816	327,579	315,320
Claims incurred	0	0	0	0
Interest credited to policyholders’ account balances	22,045	19,537	57,561	56,848
Commissions for acquiring and servicing policies	42,023	39,813	120,646	118,724
Other operating expenses	46,873	45,467	142,520	144,606
Change in deferred policy acquisition costs	(5,080)	(4,458)	(19,120)	(18,150)
Total benefits, losses and expenses	219,513	220,175	629,186	617,348
Income before federal income tax and other items	8,414	4,628	45,967	28,505
Annuity
Segment Reporting Information [Line Items]
Premiums	41,305	47,296	137,434	185,140
Other policy revenues	4,338	3,580	12,878	11,095
Net investment income	141,684	174,771	487,750	436,961
Net realized investment gains	0	0	0	0
Net gains on equity securities	0	0	0	0
Other income	653	624	1,916	1,980
Total premiums and other revenues	187,980	226,271	639,978	635,176
Policyholder benefits	59,699	64,153	191,248	231,002
Claims incurred	0	0	0	0
Interest credited to policyholders’ account balances	84,737	113,881	314,142	252,846
Commissions for acquiring and servicing policies	13,368	18,515	63,373	78,874
Other operating expenses	12,264	11,350	38,087	34,522
Change in deferred policy acquisition costs	7,006	(1,376)	(383)	(19,060)
Total benefits, losses and expenses	177,074	206,523	606,467	578,184
Income before federal income tax and other items	10,906	19,748	33,511	56,992
Health
Segment Reporting Information [Line Items]
Premiums	40,676	45,154	121,581	135,039
Other policy revenues	0	0	0	0
Net investment income	2,381	2,233	7,177	6,850
Net realized investment gains	0	0	0	0
Net gains on equity securities	0	0	0	0
Other income	5,021	6,631	15,940	18,597
Total premiums and other revenues	48,078	54,018	144,698	160,486
Policyholder benefits	0	0	0	0
Claims incurred	28,567	29,751	81,042	90,201
Interest credited to policyholders’ account balances	0	0	0	0
Commissions for acquiring and servicing policies	7,405	8,516	22,975	23,658
Other operating expenses	9,504	10,829	31,203	31,277
Change in deferred policy acquisition costs	258	466	919	2,060
Total benefits, losses and expenses	45,734	49,562	136,139	147,196
Income before federal income tax and other items	2,344	4,456	8,559	13,290
Property & Casualty
Segment Reporting Information [Line Items]
Premiums	382,784	374,842	1,125,704	1,086,862
Other policy revenues	0	0	0	0
Net investment income	16,357	15,629	48,085	46,983
Net realized investment gains	0	0	0	0
Net gains on equity securities	0	0	0	0
Other income	3,137	3,399	8,689	7,726
Total premiums and other revenues	402,278	393,870	1,182,478	1,141,571
Policyholder benefits	0	0	0	0
Claims incurred	280,695	272,885	790,447	795,501
Interest credited to policyholders’ account balances	0	0	0	0
Commissions for acquiring and servicing policies	65,893	72,135	201,635	212,156
Other operating expenses	50,111	45,277	151,677	138,244
Change in deferred policy acquisition costs	(636)	(3,426)	(3,807)	(10,726)
Total benefits, losses and expenses	396,063	386,871	1,139,952	1,135,175
Income before federal income tax and other items	6,215	6,999	42,526	6,396
Corporate & Other
Segment Reporting Information [Line Items]
Premiums	0	0	0	0
Other policy revenues	0	0	0	0
Net investment income	24,237	27,024	59,051	65,423
Net realized investment gains	31,933	(1,276)	24,975	4,775
Net gains on equity securities	8,589	126,495	282,026	150,487
Other income	1,478	1,031	4,511	3,911
Total premiums and other revenues	66,237	153,274	370,563	224,596
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	9,750	5,838	28,158	24,453
Change in deferred policy acquisition costs	0	0	0	0
Total benefits, losses and expenses	9,750	5,838	28,158	24,453
Income before federal income tax and other items	$ 56,487	$ 147,436	$ 342,405	$ 200,143