Stock-Based Compensation - Analysis of Shares Available for Grant (Detail)	12 Months Ended
	Dec. 31, 2017 shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Activity by Share-based Payment Award
Balance at January 1 (in shares)	5,088,461	6,872,282	8,458,593
Options expired and forfeited during year (in shares)	26,488	8,518	90,371
Restricted stock expired and forfeited during year (in shares)	46,500	0	89,745
Balance at December 31 (in shares)	2,964,320	5,088,461	6,872,282
Stock Options
Activity by Share-based Payment Award
Options granted during year (in shares)	(1,328,513)	(1,306,306)	(1,334,514)
Torchmark Corporation 2011 Incentive Plan | Restricted Stock, Restricted Stock Units And Performance Shares
Activity by Share-based Payment Award
Restricted stock, restricted stock units, and performance shares granted under the Torchmark Corporation 2011 Incentive Plan (in shares)	(868,616)	(486,033)	(431,913)
Minimum
Activity by Share-based Payment Award
Ratio by which each grant of stock options reduces shares available for grant	0.85
Ratio by which each grant of restricted stock reduces shares available for options	3.1
Maximum
Activity by Share-based Payment Award
Ratio by which each grant of stock options reduces shares available for grant	1
Ratio by which each grant of restricted stock reduces shares available for options	3.88