Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue:
Premium	$ 968,551	$ 961,817	$ 953,702	$ 929,835	$ 906,684	$ 899,993	$ 897,484	$ 890,973	$ 3,813,905	$ 3,595,134	$ 3,421,906
Net investment income	235,071	231,432	231,568	228,991	227,456	228,905	227,425	226,673	927,062	910,459	882,512
Other income									1,325	1,318	1,137
Total revenue									4,742,292	4,506,911	4,305,555
Expenses:
Policy benefits	663,123	650,976	650,816	607,969	594,538	585,692	589,362	587,757	2,572,884	2,357,349	2,275,242
Required interest on reserves									0	0	0
Required interest on DAC									0	0	0
Amortization of acquisition costs	144,930	140,843	146,160	143,837	139,290	138,449	138,165	135,822	575,770	551,726	516,690
Commissions, premium taxes, and non-deferred acquisition costs									304,841	298,047	278,487
Insurance administrative expense									254,932	249,079	227,531
Parent expense									10,214	10,903	12,262
Stock-based compensation expense									35,892	44,843	39,792
Interest expense									86,704	84,306	90,076
Total benefits and expenses									3,841,237	3,596,253	3,440,080
Subtotal									901,055	910,658	865,475
Non-operating items									4,308	9,401	5,168
Measure of segment profitability (pretax)									905,363	920,059	870,643
Realized gain (loss)—investments									(3,737)	20,621	9,274
Realized loss on redemption of debt									(634)	0	(11,078)
Administrative settlements										(400)	(3,590)
Legal proceedings									(3,275)	(8,358)
Non-operating expenses									(1,033)	(643)	(1,578)
Income before income taxes	$ 249,984	$ 231,538	$ 212,241	$ 202,921	$ 227,088	$ 247,330	$ 228,760	$ 228,101	896,684	931,279	863,671
American Income
Revenue:
Premium									1,363,460	1,259,942	1,174,646
Direct to Consumer
Revenue:
Premium									983,486	933,100	905,232
Liberty National
Revenue:
Premium									482,732	475,129	470,256
United American
Revenue:
Premium									462,672	427,157	392,539
Other
Revenue:
Premium									200,281	201,794	202,457
Family Heritage
Revenue:
Premium									321,274	298,012	276,776
Life
Revenue:
Premium									2,672,804	2,517,784	2,406,555
Life | American Income
Revenue:
Premium									1,257,726	1,160,495	1,081,333
Life | Direct to Consumer
Revenue:
Premium									906,959	855,543	828,935
Life | Liberty National
Revenue:
Premium									293,897	285,551	278,878
Life | United American
Revenue:
Premium									9,688	10,571	11,451
Life | Other
Revenue:
Premium									200,281	201,794	202,457
Life | Family Heritage
Revenue:
Premium									4,253	3,830	3,501
Health
Revenue:
Premium									1,141,097	1,077,346	1,015,339
Health | American Income
Revenue:
Premium									105,734	99,447	93,313
Health | Direct to Consumer
Revenue:
Premium									76,527	77,557	76,297
Health | Liberty National
Revenue:
Premium									188,835	189,578	191,378
Health | United American
Revenue:
Premium									452,980	416,582	381,076
Health | Other
Revenue:
Premium									0	0	0
Health | Family Heritage
Revenue:
Premium									317,021	294,182	273,275
Annuity
Revenue:
Premium									4	4	12
Annuity | American Income
Revenue:
Premium									0	0	0
Annuity | Direct to Consumer
Revenue:
Premium									0	0	0
Annuity | Liberty National
Revenue:
Premium									0	0	0
Annuity | United American
Revenue:
Premium									4	4	12
Annuity | Other
Revenue:
Premium									0	0	0
Annuity | Family Heritage
Revenue:
Premium									0	0	0
Operating Segments | Life
Revenue:
Premium									2,672,804	2,517,784	2,406,555
Net investment income									0	0	0
Other income									0	0	0
Total revenue									2,672,804	2,517,784	2,406,555
Expenses:
Policy benefits									1,809,373	1,638,053	1,591,790
Required interest on reserves									(698,112)	(666,168)	(636,040)
Required interest on DAC									210,152	202,502	194,297
Amortization of acquisition costs									463,586	436,881	414,200
Commissions, premium taxes, and non-deferred acquisition costs									212,859	203,052	190,007
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total benefits and expenses									1,997,858	1,814,320	1,754,254
Subtotal									674,946	703,464	652,301
Measure of segment profitability (pretax)									674,946	703,464	652,301
Operating Segments | Health
Revenue:
Premium									1,141,097	1,077,346	1,015,339
Net investment income									0	0	0
Other income									0	0	0
Total revenue									1,141,097	1,077,346	1,015,339
Expenses:
Policy benefits									733,481	687,764	649,188
Required interest on reserves									(93,475)	(87,289)	(83,243)
Required interest on DAC									26,586	25,435	24,412
Amortization of acquisition costs									110,177	112,825	100,376
Commissions, premium taxes, and non-deferred acquisition costs									91,959	94,973	88,553
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total benefits and expenses									868,728	833,708	779,286
Subtotal									272,369	243,638	236,053
Measure of segment profitability (pretax)									272,369	243,638	236,053
Operating Segments | Annuity
Revenue:
Premium									4	4	12
Net investment income									0	0	0
Other income									0	0	0
Total revenue									4	4	12
Expenses:
Policy benefits									30,030	31,532	34,264
Required interest on reserves									(41,413)	(43,522)	(47,357)
Required interest on DAC									328	494	589
Amortization of acquisition costs									2,007	2,020	2,114
Commissions, premium taxes, and non-deferred acquisition costs									23	22	26
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total benefits and expenses									(9,025)	(9,454)	(10,364)
Subtotal									9,029	9,458	10,376
Measure of segment profitability (pretax)									9,029	9,458	10,376
Operating Segments | Investment
Revenue:
Premium									0	0	0
Net investment income									927,062	910,459	882,512
Other income									0	0	0
Total revenue									927,062	910,459	882,512
Expenses:
Policy benefits									0	0	0
Required interest on reserves									833,000	796,979	766,640
Required interest on DAC									(237,066)	(228,431)	(219,298)
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									86,704	84,306	90,076
Total benefits and expenses									682,638	652,854	637,418
Subtotal									244,424	257,605	245,094
Measure of segment profitability (pretax)									244,424	257,605	245,094
Corporate & Other
Revenue:
Premium									0	0	0
Net investment income									0	0	0
Other income									1,325	1,318	1,236
Total revenue									1,325	1,318	1,236
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									250,947	240,321	223,941
Parent expense									9,891	10,260	10,684
Stock-based compensation expense									35,892	44,843	39,792
Interest expense									0	0	0
Total benefits and expenses									296,730	295,424	274,417
Subtotal									(295,405)	(294,106)	(273,181)
Measure of segment profitability (pretax)									(295,405)	(294,106)	(273,181)
Adjustments
Revenue:
Premium									0	0	0
Net investment income									0	0	0
Other income									0	0	(99)
Total revenue									0	0	(99)
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	(99)
Insurance administrative expense									3,985	8,758	3,590
Parent expense									323	643	1,578
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total benefits and expenses									4,308	9,401	5,069
Subtotal									(4,308)	(9,401)	(5,168)
Non-operating items									4,308	9,401	5,168
Measure of segment profitability (pretax)									$ 0	$ 0	$ 0