Selected Quarterly Data (Unaudited) - Schedule of Selected Quarterly Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Premium and policy charges	$ 750,611	$ 750,998	$ 765,851	$ 784,814	$ 732,545	$ 699,860	$ 705,582	$ 718,475	$ 3,052,274	$ 2,856,462	$ 2,656,318
Net investment income	178,284	176,656	177,964	176,839	174,947	169,400	175,176	174,121	709,743	693,644	693,028
Realized investment gains (losses)	1,525	4,459	5,913	(3,907)	20,883	7,283	4,661	5,006	7,990	37,833	25,904
Total revenues	930,594	932,789	950,339	958,216	928,698	877,100	885,795	897,923	3,771,938	3,589,516	3,377,401
Policy benefits	495,561	516,783	524,499	552,003	479,109	479,119	484,807	512,647	2,088,846	1,955,682	1,793,276
Amortization of acquisition expenses	100,743	98,444	102,488	101,714	98,052	94,016	96,601	96,498	403,389	385,167	364,583
Pretax income	206,429	190,850	192,784	173,063	220,587	188,791	186,380	170,235	763,126	765,993	723,782
Net income	$ 142,817	$ 132,122	$ 133,901	$ 119,632	$ 150,987	$ 130,672	$ 128,988	$ 118,677	$ 528,472	$ 529,324	$ 497,161
Basic net income per common share	$ 1.59	$ 1.45	$ 1.45	$ 1.28	$ 1.60	$ 1.37	$ 1.33	$ 1.19	$ 5.76	$ 5.48	$ 4.59
Diluted net income per common share	$ 1.56	$ 1.43	$ 1.44	$ 1.27	$ 1.58	$ 1.36	$ 1.32	$ 1.17	$ 5.68	$ 5.41	$ 4.53