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
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Premiums	$ 536,237	$ 544,980
Other policy revenues	74,248	71,339
Net investment income	292,346	208,669
Net realized investment gains	2,947	1,044
Net gains on equity securities	206,377	(33,170)
Other income	11,538	10,513
Total premiums and other revenues	1,123,693	803,375
Policyholder benefits	168,226	183,292
Claims incurred	263,911	270,630
Interest credited to policyholders’ account balances	141,234	70,545
Commissions for acquiring and servicing policies	138,645	144,696
Other operating expenses	133,610	130,394
Change in deferred policy acquisition costs	(6,631)	(16,966)
Total benefits, losses and expenses	838,995	782,591
Income before federal income tax and other items	284,698	20,784
Life
Segment Reporting Information [Line Items]
Premiums	86,468	81,376
Other policy revenues	70,244	67,731
Net investment income	68,756	57,768
Net realized investment gains	0	0
Net gains on equity securities	0	0
Other income	676	755
Total premiums and other revenues	226,144	207,630
Policyholder benefits	109,465	98,546
Claims incurred	0	0
Interest credited to policyholders’ account balances	20,319	16,265
Commissions for acquiring and servicing policies	37,742	39,520
Other operating expenses	48,978	50,950
Change in deferred policy acquisition costs	(4,835)	(6,443)
Total benefits, losses and expenses	211,669	198,838
Income before federal income tax and other items	14,475	8,792
Annuity
Segment Reporting Information [Line Items]
Premiums	39,907	70,616
Other policy revenues	4,004	3,608
Net investment income	190,711	113,480
Net realized investment gains	0	0
Net gains on equity securities	0	0
Other income	689	725
Total premiums and other revenues	235,311	188,429
Policyholder benefits	58,761	84,746
Claims incurred	0	0
Interest credited to policyholders’ account balances	120,915	54,280
Commissions for acquiring and servicing policies	26,866	30,004
Other operating expenses	12,474	11,319
Change in deferred policy acquisition costs	(3,020)	(8,873)
Total benefits, losses and expenses	215,996	171,476
Income before federal income tax and other items	19,315	16,953
Health
Segment Reporting Information [Line Items]
Premiums	38,681	41,015
Other policy revenues	0	0
Net investment income	2,420	2,354
Net realized investment gains	0	0
Net gains on equity securities	0	0
Other income	5,385	5,157
Total premiums and other revenues	46,486	48,526
Policyholder benefits	0	0
Claims incurred	25,767	28,140
Interest credited to policyholders’ account balances	0	0
Commissions for acquiring and servicing policies	6,878	6,016
Other operating expenses	10,992	10,358
Change in deferred policy acquisition costs	807	1,088
Total benefits, losses and expenses	44,444	45,602
Income before federal income tax and other items	2,042	2,924
Property & Casualty
Segment Reporting Information [Line Items]
Premiums	371,181	351,973
Other policy revenues	0	0
Net investment income	15,022	15,861
Net realized investment gains	0	0
Net gains on equity securities	0	0
Other income	2,722	2,063
Total premiums and other revenues	388,925	369,897
Policyholder benefits	0	0
Claims incurred	238,144	242,490
Interest credited to policyholders’ account balances	0	0
Commissions for acquiring and servicing policies	67,159	69,156
Other operating expenses	51,885	47,801
Change in deferred policy acquisition costs	417	(2,738)
Total benefits, losses and expenses	357,605	356,709
Income before federal income tax and other items	31,320	13,188
Corporate & Other
Segment Reporting Information [Line Items]
Premiums	0	0
Other policy revenues	0	0
Net investment income	15,437	19,206
Net realized investment gains	2,947	1,044
Net gains on equity securities	206,377	(33,170)
Other income	2,066	1,813
Total premiums and other revenues	226,827	(11,107)
Policyholder benefits	0	0
Claims incurred	0	0
Interest credited to policyholders’ account balances	0	0
Commissions for acquiring and servicing policies	0	0
Other operating expenses	9,281	9,966
Change in deferred policy acquisition costs	0	0
Total benefits, losses and expenses	9,281	9,966
Income before federal income tax and other items	$ 217,546	$ (21,073)