Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Premium	$ 750,998	$ 699,860	$ 2,301,663	$ 2,123,917
Net investment income	176,656	169,400	531,459	518,697
Other income	676	557	1,757	1,254
Total revenue			2,834,879	2,643,868
Policy benefits	516,783	479,119	1,593,285	1,476,573
Required interest on policy reserves			0	0
Required interest on deferred acquisition costs			0	0
Amortization of acquisition costs	98,444	94,016	302,646	287,115
Commissions, premium taxes, and non-deferred acquisition costs	53,791	46,898	166,490	148,254
Insurance administrative expense			134,235	120,942
Parent expense			7,234	6,818
Stock compensation expense			19,376	16,547
Interest expense	19,676	19,843	61,381	59,163
Total benefits and expenses	741,939	688,309	2,284,647	2,115,412
Subtotal			550,232	528,456
Nonoperating items			21,334	19,537
Amortization of low-income housing			18,556	16,826
Measure of segment profitability (pretax)			590,122	564,819
Deduct applicable income taxes			(193,109)	(184,800)
Segment profits after tax			397,013	380,019
Add back income taxes applicable to segment profitability			193,109	184,800
Add (deduct) realized investment gains (losses)			6,465	16,950
Deduct Part D adjustment			(12,573)	(18,922)
Deduct amortization of low-income housing			(18,556)	(16,826)
Deduct Guaranty Fund Assessment			(1,155)
Deduct Family Heritage Life acquisition expense		(615)		(615)
Deduct legal settlement expenses			(9,125)
Income before income taxes	190,850	188,791	556,697	545,406
Family Heritage Adjustment [Member]
Segment Reporting Information [Line Items]
Amortization of acquisition costs			1,519
Operating Segments [Member] | Life Insurance [Member]
Segment Reporting Information [Line Items]
Premium			1,416,859	1,356,527
Total revenue			1,416,859	1,356,527
Policy benefits			921,921	879,317
Required interest on policy reserves			(379,163)	(360,454)
Required interest on deferred acquisition costs			123,599	122,453
Amortization of acquisition costs			243,259	231,926
Commissions, premium taxes, and non-deferred acquisition costs			99,398	102,696
Total benefits and expenses			1,009,014	975,938
Subtotal			407,845	380,589
Measure of segment profitability (pretax)			407,845	380,589
Operating Segments [Member] | Health Insurance [Member]
Segment Reporting Information [Line Items]
Premium			875,697	760,825
Total revenue			875,697	760,825
Policy benefits			609,067	539,212
Required interest on policy reserves			(44,602)	(28,242)
Required interest on deferred acquisition costs			17,503	13,710
Amortization of acquisition costs			54,051	47,536
Commissions, premium taxes, and non-deferred acquisition costs			67,259	45,752
Total benefits and expenses			703,278	617,968
Subtotal			172,419	142,857
Measure of segment profitability (pretax)			172,419	142,857
Operating Segments [Member] | Annuity [Member]
Segment Reporting Information [Line Items]
Premium			401	440
Total revenue			401	440
Policy benefits			32,393	32,997
Required interest on policy reserves			(43,298)	(44,690)
Required interest on deferred acquisition costs			1,399	1,721
Amortization of acquisition costs			6,855	7,653
Commissions, premium taxes, and non-deferred acquisition costs			46	53
Total benefits and expenses			(2,605)	(2,266)
Subtotal			3,006	2,706
Measure of segment profitability (pretax)			3,006	2,706
Operating Segments [Member] | Investment [Member]
Segment Reporting Information [Line Items]
Net investment income			550,015	535,325
Total revenue			550,015	535,325
Required interest on policy reserves			467,063	433,386
Required interest on deferred acquisition costs			(142,501)	(137,884)
Interest expense			61,381	58,965
Total benefits and expenses			385,943	354,467
Subtotal			164,072	180,858
Measure of segment profitability (pretax)			164,072	180,858
Operating Segments [Member] | Other and Corporate [Member]
Segment Reporting Information [Line Items]
Other income			1,970	1,501
Total revenue			1,970	1,501
Insurance administrative expense			133,080	120,942
Parent expense			6,734	6,203
Stock compensation expense			19,376	16,547
Total benefits and expenses			159,190	143,692
Subtotal			(157,220)	(142,191)
Measure of segment profitability (pretax)			(157,220)	(142,191)
Adjustments [Member] | Adjustments [Member]
Segment Reporting Information [Line Items]
Premium			8,706	6,125
Net investment income			(18,556)	(16,628)
Other income			(213)	(247)
Total revenue			(10,063)	(10,750)
Policy benefits			29,904	25,047
Amortization of acquisition costs			(1,519)
Commissions, premium taxes, and non-deferred acquisition costs			(213)	(247)
Insurance administrative expense			1,155
Parent expense			500	615
Interest expense				198
Total benefits and expenses			29,827	25,613
Subtotal			(39,890)	(36,363)
Nonoperating items			21,334	19,537
Amortization of low-income housing			18,556	16,826
Measure of segment profitability (pretax)			0	0
Deduct amortization of low-income housing			$ (18,556)	$ (16,826)