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, 2013	Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012
Segment Reporting Information [Line Items]
Premium	$ 765,851	$ 705,582	$ 1,550,665		$ 1,424,057
Net investment income	177,964	175,176	354,803		349,297
Other income	611	376	1,081		697
Total revenue			1,906,549		1,774,051
Policy benefits	524,499	484,807	1,076,502		997,454
Policy reserves			0		0
Deferred acquisition costs			0		0
Amortization of acquisition costs	102,488	96,601	204,202		193,099
Commissions, premium taxes, and non-deferred acquisition costs	54,448	50,600	112,699		101,356
Insurance administrative expense			89,217	[1]	80,427	[1]
Parent expense			5,427		4,210
Stock compensation expense			12,956		11,237
Interest expense	20,828	19,649	41,705		39,320
Total benefits and expenses	757,555	699,415	1,542,708		1,427,103
Subtotal			363,841		346,948
Nonoperating items			16,705		21,366
Amortization of low-income housing			12,401	[2]	11,164	[2]
Measure of segment profitability (pretax)			392,947		379,478
Deduct applicable income taxes			(128,632)		(124,209)
Segment profits after tax			264,315		255,269
Add back income taxes applicable to segment profitability			128,632		124,209
Add (deduct) realized investment gains (losses)			2,006		9,667
Deduct Part D adjustment			(15,050)	[3]	(21,366)	[4]
Deduct amortization of low-income housing			(12,401)	[2]	(11,164)	[2]
Deduct Guaranty Fund Assessment			(1,155)	[5]
Deduct legal settlement expense			(500)	[6]
Income before income taxes	192,784	186,380	365,847		356,615
Life Insurance
Segment Reporting Information [Line Items]
Premium			945,923		902,828
Total revenue			945,923		902,828
Policy benefits			615,990		583,472
Policy reserves			(251,356)		(238,656)
Deferred acquisition costs			82,340		81,814
Amortization of acquisition costs			162,534		155,649
Commissions, premium taxes, and non-deferred acquisition costs			67,575		70,254
Total benefits and expenses			677,083		652,533
Subtotal			268,840		250,295
Measure of segment profitability (pretax)			268,840		250,295
Health Insurance
Segment Reporting Information [Line Items]
Premium			589,560		509,676
Total revenue			589,560		509,676
Policy benefits			409,043		359,489
Policy reserves			(29,372)		(18,700)
Deferred acquisition costs			11,504		9,196
Amortization of acquisition costs			36,785		32,243
Commissions, premium taxes, and non-deferred acquisition costs			45,240		31,234
Total benefits and expenses			473,200		413,462
Subtotal			116,360		96,214
Measure of segment profitability (pretax)			116,360		96,214
Annuity
Segment Reporting Information [Line Items]
Premium			287		279
Total revenue			287		279
Policy benefits			21,524		21,853
Policy reserves			(29,105)		(29,766)
Deferred acquisition costs			959		1,177
Amortization of acquisition costs			4,883		5,207
Commissions, premium taxes, and non-deferred acquisition costs			30		36
Total benefits and expenses			(1,709)		(1,493)
Subtotal			1,996		1,772
Measure of segment profitability (pretax)			1,996		1,772
Investment
Segment Reporting Information [Line Items]
Net investment income			367,204		360,329
Total revenue			367,204		360,329
Policy reserves			309,833		287,122
Deferred acquisition costs			(94,803)		(92,187)
Interest expense			41,705		39,188
Total benefits and expenses			256,735		234,123
Subtotal			110,469		126,206
Measure of segment profitability (pretax)			110,469		126,206
Other & Corporate
Segment Reporting Information [Line Items]
Other income			1,227		865
Total revenue			1,227		865
Insurance administrative expense			88,062	[1]	80,427	[1]
Parent expense			4,927		4,210
Stock compensation expense			12,956		11,237
Total benefits and expenses			105,945		95,874
Subtotal			(104,718)		(95,009)
Measure of segment profitability (pretax)			(104,718)		(95,009)
Adjustments
Segment Reporting Information [Line Items]
Premium			14,895	[4]	11,274	[4]
Net investment income			(12,401)	[2]	(11,032)	[2],[7]
Other income			(146)	[3]	(168)	[3]
Total revenue			2,348		74
Policy benefits			29,945	[4]	32,640	[4]
Commissions, premium taxes, and non-deferred acquisition costs			(146)	[3]	(168)	[3]
Insurance administrative expense			1,155	[1],[5]
Parent expense			500	[6]
Interest expense					132	[7]
Total benefits and expenses			31,454		32,604
Subtotal			(29,106)		(32,530)
Nonoperating items			16,705		21,366
Amortization of low-income housing			12,401	[2]	11,164	[2]
Measure of segment profitability (pretax)			0		0
Deduct amortization of low-income housing			$ (12,401)	[2]	$ (11,164)	[2]

[1] Administrative expense is not allocated to insurance segments.

[2] Amortization of low-income housing expense, considered a component of income tax expense in the segment analysis.

[3] Elimination of intersegment commission.

[4] Medicare Part D items adjusted to GAAP from the segment analysis, which matches expected benefits with policy premium.

[5] Guaranty Fund Assessment.

[6] Legal settlement expense.

[7] Reclassification of interest amount due to accounting rule requiring deconsolidation of Trust Preferred Securities. Management views the Trust Preferreds as consolidated debt.