Business Segments - Schedule of Segment Premium Revenue by Each Marketing Groups (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Amount	$ 3,282,935	$ 3,137,034	$ 2,998,720
% of Total	100.00%	100.00%	100.00%
United American Independent
Segment Reporting Information [Line Items]
Amount	$ 376,690	$ 368,786	$ 360,501
% of Total	12.00%	12.00%	12.00%
Liberty National Exclusive
Segment Reporting Information [Line Items]
Amount	$ 470,842	$ 472,274	$ 480,263
% of Total	14.00%	15.00%	16.00%
American Income Exclusive
Segment Reporting Information [Line Items]
Amount	$ 1,088,315	$ 997,737	$ 911,242
% of Total	33.00%	32.00%	30.00%
Family Heritage Exclusive
Segment Reporting Information [Line Items]
Amount	$ 256,727	$ 238,941	$ 223,425
% of Total	8.00%	8.00%	8.00%
Direct Response
Segment Reporting Information [Line Items]
Amount	$ 886,375	$ 853,158	$ 816,303
% of Total	27.00%	27.00%	27.00%
Other
Segment Reporting Information [Line Items]
Amount	$ 203,986	$ 206,138	$ 206,986
% of Total	6.00%	6.00%	7.00%
Life
Segment Reporting Information [Line Items]
Amount	$ 2,306,547	$ 2,189,333	$ 2,073,065
% of Total	100.00%	100.00%	100.00%
Life | United American Independent
Segment Reporting Information [Line Items]
Amount	$ 12,547	$ 13,733	$ 15,036
% of Total	1.00%	1.00%	1.00%
Life | Liberty National Exclusive
Segment Reporting Information [Line Items]
Amount	$ 274,635	$ 270,476	$ 271,113
% of Total	12.00%	12.00%	13.00%
Life | American Income Exclusive
Segment Reporting Information [Line Items]
Amount	$ 999,279	$ 913,355	$ 830,903
% of Total	43.00%	42.00%	40.00%
Life | Family Heritage Exclusive
Segment Reporting Information [Line Items]
Amount	$ 3,193	$ 2,866	$ 2,334
% of Total	0.00%	0.00%	0.00%
Life | Direct Response
Segment Reporting Information [Line Items]
Amount	$ 812,907	$ 782,765	$ 746,693
% of Total	35.00%	36.00%	36.00%
Life | Other
Segment Reporting Information [Line Items]
Amount	$ 203,986	$ 206,138	$ 206,986
% of Total	9.00%	9.00%	10.00%
Health
Segment Reporting Information [Line Items]
Amount	$ 976,373	$ 947,663	$ 925,520
% of Total	100.00%	100.00%	100.00%
Health | United American Independent
Segment Reporting Information [Line Items]
Amount	$ 364,128	$ 355,015	$ 345,330
% of Total	37.00%	38.00%	37.00%
Health | Liberty National Exclusive
Segment Reporting Information [Line Items]
Amount	$ 196,207	$ 201,798	$ 209,150
% of Total	20.00%	21.00%	23.00%
Health | American Income Exclusive
Segment Reporting Information [Line Items]
Amount	$ 89,036	$ 84,382	$ 80,339
% of Total	9.00%	9.00%	9.00%
Health | Family Heritage Exclusive
Segment Reporting Information [Line Items]
Amount	$ 253,534	$ 236,075	$ 221,091
% of Total	26.00%	25.00%	24.00%
Health | Direct Response
Segment Reporting Information [Line Items]
Amount	$ 73,468	$ 70,393	$ 69,610
% of Total	8.00%	7.00%	7.00%
Annuity
Segment Reporting Information [Line Items]
Amount	$ 15	$ 38	$ 135
% of Total	100.00%	100.00%	100.00%
Annuity | United American Independent
Segment Reporting Information [Line Items]
Amount	$ 15	$ 38	$ 135
% of Total	100.00%	100.00%	100.00%