Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (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
Revenue:
Premium	$ 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
Other income									1,318	1,137	1,142
Total revenue									4,506,911	4,305,555	4,131,962
Expenses:
Policy 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
Required interest on reserves									0	0	0
Required interest on DAC									0	0	0
Amortization of acquisition costs	139,290	138,449	138,165	135,822	128,501	129,492	129,077	129,620	551,726	516,690	490,403
Commissions, premium taxes, and non-deferred acquisition costs									298,047	278,487	264,860
Insurance administrative expense									249,079	227,531	210,590
Parent expense									10,903	12,262	9,631
Stock-based compensation expense									44,843	39,792	37,034
Interest expense									84,306	90,076	84,532
Total expenses									3,596,253	3,440,080	3,324,925
Subtotal									910,658	865,475	807,037
Non-operating items									9,401	5,168	14,128
Measure of segment profitability (pretax)									920,059	870,643	821,165
Realized gains (losses) from investments									20,621	9,274	27,652
Realized loss—redemption of debt									0	(11,078)	(4,041)
Administrative settlements									(400)	(3,590)	(8,659)
Legal proceedings									(8,358)
Non-operating fees									(643)	(1,578)	(288)
Guaranty fund assessments											(1,801)
Stock-based compensation expense—Tax reform adjustment											(3,380)
Income before income taxes	$ 227,088	$ 247,330	$ 228,760	$ 228,101	$ 203,635	$ 220,330	$ 226,864	$ 212,842	931,279	863,671	830,648
Administrative settlements, net of tax											(5,600)
Guaranty fund assessments, net of tax											(1,200)
Stock-based compensation expense—Tax reform adjustment, net of tax											(2,200)
Operating Segments | Life
Revenue:
Premium									2,517,784	2,406,555	2,306,547
Net investment income									0	0	0
Other income									0	0	0
Total revenue									2,517,784	2,406,555	2,306,547
Expenses:
Policy benefits									1,638,053	1,591,790	1,549,602
Required interest on reserves									(666,168)	(636,040)	(607,007)
Required interest on DAC									202,502	194,297	186,236
Amortization of acquisition costs									436,881	414,200	396,268
Commissions, premium taxes, and non-deferred acquisition costs									203,052	190,007	177,111
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total expenses									1,814,320	1,754,254	1,702,210
Subtotal									703,464	652,301	604,337
Measure of segment profitability (pretax)									703,464	652,301	604,337
Operating Segments | Health
Revenue:
Premium									1,077,346	1,015,339	976,373
Net investment income									0	0	0
Other income									0	0	0
Total revenue									1,077,346	1,015,339	976,373
Expenses:
Policy benefits									687,764	649,188	628,640
Required interest on reserves									(87,289)	(83,243)	(77,792)
Required interest on DAC									25,435	24,412	23,454
Amortization of acquisition costs									112,825	100,376	96,519
Commissions, premium taxes, and non-deferred acquisition costs									94,973	88,553	86,044
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total expenses									833,708	779,286	756,865
Subtotal									243,638	236,053	219,508
Measure of segment profitability (pretax)									243,638	236,053	219,508
Operating Segments | Annuity
Revenue:
Premium									4	12	15
Net investment income									0	0	0
Other income									0	0	0
Total revenue									4	12	15
Expenses:
Policy benefits									31,532	34,264	35,836
Required interest on reserves									(43,522)	(47,357)	(49,571)
Required interest on DAC									494	589	690
Amortization of acquisition costs									2,020	2,114	2,466
Commissions, premium taxes, and non-deferred acquisition costs									22	26	32
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total expenses									(9,454)	(10,364)	(10,547)
Subtotal									9,458	10,376	10,562
Measure of segment profitability (pretax)									9,458	10,376	10,562
Operating Segments | Investment
Revenue:
Premium									0	0	0
Net investment income									910,459	882,512	847,885
Other income									0	0	0
Total revenue									910,459	882,512	847,885
Expenses:
Policy benefits									0	0	0
Required interest on reserves									796,979	766,640	734,370
Required interest on DAC									(228,431)	(219,298)	(210,380)
Amortization of acquisition costs									0	0	0
Commissions, premium taxes, and non-deferred acquisition costs									0	0	0
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									84,306	90,076	84,532
Total expenses									652,854	637,418	608,522
Subtotal									257,605	245,094	239,363
Measure of segment profitability (pretax)									257,605	245,094	239,363
Corporate & Other
Revenue:
Premium									0	0	0
Net investment income									0	0	0
Other income									1,318	1,236	1,270
Total revenue									1,318	1,236	1,270
Expenses:
Policy benefits									0	0	0
Required interest on reserves									0	0	0
Required interest on DAC									0	0	0
Amortization of acquisition costs									0	0	0
Commissions, premium taxes, and non-deferred acquisition costs									0	0	0
Insurance administrative expense									240,321	223,941	210,590
Parent expense									10,260	10,684	9,631
Stock-based compensation expense									44,843	39,792	33,654
Interest expense									0	0	0
Total expenses									295,424	274,417	253,875
Subtotal									(294,106)	(273,181)	(252,605)
Measure of segment profitability (pretax)									(294,106)	(273,181)	(252,605)
Adjustments
Revenue:
Other income										(99)	(128)
Total revenue									0	(99)	(128)
Expenses:
Policy benefits											13,797
Amortization of acquisition costs											(4,850)
Commissions, premium taxes, and non-deferred acquisition costs										(99)	1,673
Insurance administrative expense									8,758	3,590
Parent expense									643	1,578
Stock-based compensation expense											3,380
Total expenses									9,401	5,069	14,000
Subtotal									(9,401)	(5,168)	(14,128)
Non-operating items									9,401	$ 5,168	14,128
Measure of segment profitability (pretax)									$ 0		$ 0