Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Premium	$ 799,866	$ 765,851	$ 1,618,886	$ 1,550,665
Net investment income	182,877	177,964	363,877	354,803
Other income	663	611	1,144	1,081
Total revenue			1,983,907	1,906,549
Policy benefits	555,207	524,499	1,141,724	1,076,502
Policy reserves			0	0
Deferred acquisition costs			0	0
Amortization of acquisition costs	104,561	102,488	209,294	204,202
Commissions, premium taxes, and non-deferred acquisition costs	62,020	54,448	121,398	112,699
Insurance administrative expense			91,910	89,217
Parent expense			4,045	5,427
Stock compensation expense			17,401	12,956
Interest expense	19,037	20,828	38,086	41,705
Total benefits and expenses	797,225	757,555	1,623,858	1,542,708
Subtotal			360,049	363,841
Nonoperating items			31,162	16,705
Amortization of low-income housing			14,104	12,401
Measure of segment profitability (pretax)			405,315	392,947
Deduct applicable income taxes			(132,458)	(128,632)
Segment profits after tax			272,857	264,315
Add back income taxes applicable to segment profitability			132,458	128,632
Add (deduct) realized investment gains (losses)			17,196	2,006
Deduct Part D adjustment			(25,768)	(15,050)
Deduct amortization of low-income housing			(14,104)	(12,401)
Deduct Guaranty Fund Assessment				(1,155)
Deduct legal settlement expenses			(2,337)	(500)
Deduct administrative settlements			(3,057)
Income before income taxes	186,758	192,784	377,245	365,847
Operating Segments [Member] | Life Insurance [Member]
Segment Reporting Information [Line Items]
Premium			981,010	945,923
Total revenue			981,010	945,923
Policy benefits			641,311	615,990
Policy reserves			(262,328)	(251,356)
Deferred acquisition costs			83,633	82,340
Amortization of acquisition costs			167,991	162,534
Commissions, premium taxes, and non-deferred acquisition costs			69,068	67,575
Total benefits and expenses			699,675	677,083
Subtotal			281,335	268,840
Measure of segment profitability (pretax)			281,335	268,840
Operating Segments [Member] | Health Insurance [Member]
Segment Reporting Information [Line Items]
Premium			434,533	439,751
Total revenue			434,533	439,751
Policy benefits			280,140	283,852
Policy reserves			(31,758)	(29,372)
Deferred acquisition costs			11,223	11,169
Amortization of acquisition costs			36,072	35,528
Commissions, premium taxes, and non-deferred acquisition costs			39,611	38,450
Total benefits and expenses			335,288	339,627
Subtotal			99,245	100,124
Measure of segment profitability (pretax)			99,245	100,124
Operating Segments [Member] | Medicare Part D [Member]
Segment Reporting Information [Line Items]
Premium			167,990	149,809
Total revenue			167,990	149,809
Policy benefits			135,233	125,191
Deferred acquisition costs			354	335
Amortization of acquisition costs			1,377	1,257
Commissions, premium taxes, and non-deferred acquisition costs			12,815	6,790
Total benefits and expenses			149,779	133,573
Subtotal			18,211	16,236
Measure of segment profitability (pretax)			18,211	16,236
Operating Segments [Member] | Annuity [Member]
Segment Reporting Information [Line Items]
Premium			222	287
Total revenue			222	287
Policy benefits			21,084	21,524
Policy reserves			(27,616)	(29,105)
Deferred acquisition costs			764	959
Amortization of acquisition costs			3,854	4,883
Commissions, premium taxes, and non-deferred acquisition costs			26	30
Total benefits and expenses			(1,888)	(1,709)
Subtotal			2,110	1,996
Measure of segment profitability (pretax)			2,110	1,996
Operating Segments [Member] | Investment [Member]
Segment Reporting Information [Line Items]
Net investment income			377,981	367,204
Total revenue			377,981	367,204
Policy reserves			321,702	309,833
Deferred acquisition costs			(95,974)	(94,803)
Interest expense			38,086	41,705
Total benefits and expenses			263,814	256,735
Subtotal			114,167	110,469
Measure of segment profitability (pretax)			114,167	110,469
Operating Segments [Member] | Other and Corporate [Member]
Segment Reporting Information [Line Items]
Other income			1,266	1,227
Total revenue			1,266	1,227
Insurance administrative expense			89,573	88,062
Parent expense			4,045	4,927
Stock compensation expense			17,401	12,956
Total benefits and expenses			111,019	105,945
Subtotal			(109,753)	(104,718)
Measure of segment profitability (pretax)			(109,753)	(104,718)
Adjustments [Member]
Segment Reporting Information [Line Items]
Premium			35,131	14,895
Net investment income			(14,104)	(12,401)
Other income			(122)	(146)
Total revenue			20,905	2,348
Policy benefits			63,956	29,945
Commissions, premium taxes, and non-deferred acquisition costs			(122)	(146)
Insurance administrative expense			2,337	1,155
Parent expense				500
Total benefits and expenses			66,171	31,454
Subtotal			(45,266)	(29,106)
Nonoperating items			31,162	16,705
Amortization of low-income housing			14,104	12,401
Measure of segment profitability (pretax)			0	0
Deduct amortization of low-income housing			(14,104)	(12,401)
Adjustments [Member] | Medicare Part D [Member]
Segment Reporting Information [Line Items]
Premium			35,131	14,895
Policy benefits			60,899	29,945
Deduct Part D adjustment			$ (25,768)	$ (15,050)