Schedule Of Selected Quarterly Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Data [Line Items]
Premium and policy charges	$ 732,545		$ 699,860		$ 705,582		$ 718,475		$ 653,542		$ 650,525		$ 672,350		$ 679,901		$ 2,856,462	$ 2,656,318	$ 2,651,758
Net investment income	174,947		169,400		175,176		174,121		174,786		173,491		173,104		171,647		693,644	693,028	676,364
Realized investment gains(losses)	20,883		7,283		4,661		5,006		4,755		12,600		31,272		(22,723)		37,833	25,904	37,340
Total revenues	928,698		877,100		885,795		897,923		833,554		837,241		877,334		829,272		3,589,516	3,377,401	3,367,632
Policy benefits	479,109		479,119		484,807		512,647		435,681		438,774		454,694		464,127		1,955,682	1,793,276	1,793,044
Amortization of acquisition expenses	98,052		94,016		96,601		96,498		90,557		89,899		91,664		92,463		385,167	364,583	362,390
Pretax Income from continuing operations	220,587		188,791		186,380		170,235		177,090		190,123		209,052		147,517		765,993	723,782	750,570
Income from continuing operations	150,987		130,672		128,988		118,677		122,839		131,256		142,781		100,740		529,324	497,616	504,095
Income from discontinued operations	0		0		0		0		0		144		0		(599)		0	(455)	(5,229)
Net income	$ 150,987		$ 130,672		$ 128,988		$ 118,677		$ 122,839		$ 131,400		$ 142,781		$ 100,141		$ 529,324	$ 497,161	$ 498,866
Continuing operations	$ 1.60	[1]	$ 1.37	[1]	$ 1.33	[1]	$ 1.19	[1]	$ 1.21	[1]	$ 1.25	[1]	$ 1.29	[1]	$ 0.87	[1]	$ 5.48	$ 4.60	$ 4.13
Discontinued operations	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ (0.01)	[1]	$ 0.00	$ (0.01)	$ (0.04)
Total basic net income per share	$ 1.60	[1]	$ 1.37	[1]	$ 1.33	[1]	$ 1.19	[1]	$ 1.21	[1]	$ 1.25	[1]	$ 1.29	[1]	$ 0.86	[1]	$ 5.48	$ 4.59	$ 4.09
Continuing operations	$ 1.58	[1]	$ 1.36	[1]	$ 1.32	[1]	$ 1.17	[1]	$ 1.20	[1]	$ 1.25	[1]	$ 1.27	[1]	$ 0.85	[1]	$ 5.41	$ 4.53	$ 4.09
Discontinued operations	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ 0.00	[1]	$ (0.01)	[1]	$ 0.00	$ 0.00	$ (0.04)
Total diluted net income per share	$ 1.58	[1]	$ 1.36	[1]	$ 1.32	[1]	$ 1.17	[1]	$ 1.20	[1]	$ 1.25	[1]	$ 1.27	[1]	$ 0.84	[1]	$ 5.41	$ 4.53	$ 4.05

[1] Basic and diluted net income per share by quarter may not add to per share income on a year-to-date basis due to share weighting and rounding.