Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014		Jun. 30, 2015	Jun. 30, 2014
Revenue:
Premium	$ 835,567	$ 799,866	[1]	$ 1,665,715	$ 1,618,886	[1]
Net investment income	194,823	189,930	[1]	386,419	377,981	[1]
Other income	691	663	[1]	1,360	1,144	[1]
Total revenue				2,053,494	1,998,011
Expenses:
Policy benefits	592,293	555,207	[1]	1,184,190	1,141,724	[1]
Policy reserves				0	0
Deferred acquisition costs				0	0
Amortization of acquisition costs	112,904	104,561	[1]	224,167	209,294	[1]
Commissions, premium taxes, and non-deferred acquisition costs	64,018	62,020	[1]	127,286	121,398	[1]
Insurance administrative expense				94,063	91,910
Parent expense				4,485	4,045
Stock compensation expense				15,041	17,401
Interest expense	19,114	19,037	[1]	38,174	38,086	[1]
Total benefits and expenses	845,306	797,225	[1]	1,687,406	1,623,858	[1]
Subtotal				366,088	374,153
Nonoperating items				30,865	31,162
Measure of segment profitability (pretax)				396,953	405,315
Deduct applicable income taxes				(129,909)	(132,458)
Net operating income				267,044	272,857
Add (deduct) realized investment gains (losses)				2,732	17,196
Deduct Part D adjustment				(30,865)	(25,768)
Deduct legal settlement expenses					(2,337)
Deduct administrative settlements		(3,100)			(3,057)
Income before income taxes	$ 188,388	$ 193,811	[1]	368,820	391,349	[1]
Operating Segments [Member] | Life Insurance [Member]
Revenue:
Premium				1,033,380	981,010
Total revenue				1,033,380	981,010
Expenses:
Policy benefits				687,065	641,311
Policy reserves				(273,615)	(262,328)
Deferred acquisition costs				85,985	83,633
Amortization of acquisition costs				177,265	167,991
Commissions, premium taxes, and non-deferred acquisition costs				75,900	69,068
Total benefits and expenses				752,600	699,675
Subtotal				280,780	281,335
Measure of segment profitability (pretax)				280,780	281,335
Operating Segments [Member] | Health Insurance [Member]
Revenue:
Premium				461,082	434,533
Total revenue				461,082	434,533
Expenses:
Policy benefits				299,227	280,140
Policy reserves				(34,034)	(31,758)
Deferred acquisition costs				11,358	11,223
Amortization of acquisition costs				40,924	36,072
Commissions, premium taxes, and non-deferred acquisition costs				40,418	39,611
Total benefits and expenses				357,893	335,288
Subtotal				103,189	99,245
Measure of segment profitability (pretax)				103,189	99,245
Operating Segments [Member] | Medicare Part D [Member]
Revenue:
Premium				154,705	167,990
Total revenue				154,705	167,990
Expenses:
Policy benefits				$ 130,764	$ 135,233
Policy reserves
Deferred acquisition costs				$ 446	$ 354
Amortization of acquisition costs				1,769	1,377
Commissions, premium taxes, and non-deferred acquisition costs				11,049	12,815
Total benefits and expenses				144,028	149,779
Subtotal				10,677	18,211
Measure of segment profitability (pretax)				10,677	18,211
Operating Segments [Member] | Annuity [Member]
Revenue:
Premium				78	222
Total revenue				78	222
Expenses:
Policy benefits				19,799	21,084
Policy reserves				(26,756)	(27,616)
Deferred acquisition costs				607	764
Amortization of acquisition costs				4,209	3,854
Commissions, premium taxes, and non-deferred acquisition costs				23	26
Total benefits and expenses				(2,118)	(1,888)
Subtotal				2,196	2,110
Measure of segment profitability (pretax)				2,196	2,110
Operating Segments [Member] | Investment [Member]
Revenue:
Net investment income				386,419	377,981
Total revenue				386,419	377,981
Expenses:
Policy reserves				334,405	321,702
Deferred acquisition costs				(98,396)	(95,974)
Interest expense				38,174	38,086
Total benefits and expenses				274,183	263,814
Subtotal				112,236	114,167
Measure of segment profitability (pretax)				112,236	114,167
Operating Segments [Member] | Other and Corporate [Member]
Revenue:
Other income				1,464	1,266
Total revenue				1,464	1,266
Expenses:
Insurance administrative expense				94,063	89,573
Parent expense				4,485	4,045
Stock compensation expense				15,041	17,401
Total benefits and expenses				113,589	111,019
Subtotal				(112,125)	(109,753)
Measure of segment profitability (pretax)				(112,125)	(109,753)
Adjustments [Member]
Revenue:
Premium				$ 16,470	$ 35,131
Net investment income
Other income				$ (104)	$ (122)
Total revenue				16,366	35,009
Expenses:
Policy benefits					$ 63,956
Amortization of acquisition costs
Commissions, premium taxes, and non-deferred acquisition costs				$ (104)	$ (122)
Insurance administrative expense					$ 2,337
Parent expense
Total benefits and expenses				$ 47,231	$ 66,171
Subtotal				(30,865)	(31,162)
Nonoperating items				30,865	31,162
Measure of segment profitability (pretax)				0	0
Adjustments [Member] | Medicare Part D [Member]
Revenue:
Premium				16,470	35,131
Expenses:
Policy benefits				47,335	60,899
Deduct Part D adjustment				$ (30,865)	$ (25,768)

[1] Certain balances were retrospectively adjusted to give effect to the adoption of new accounting guidance as described in Note G- Adoption of New Accounting Standards