Significant Accounting Policies - Additional Information (Detail) $ in Thousands			12 Months Ended
		Dec. 22, 2017 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment			4
Percentage of investment securities not classified as corporate securities, state and municipal securities, redeemable preferred stocks and U.S. government securities (less than)			1.00%
Fair value determined by third party			95.00%
Advanced agent commissions receivable			$ 378,000	$ 353,000
Advertising costs charged to earnings and included in other operating expense			9,300	9,300	$ 9,700
Capitalized advertising costs			1,280,000	1,250,000
Carrying value of investment in low-income housing interests			228,000	280,000
Obligations under future commitments for low-income housing interests			34,000
Original cost of property and equipment			217,000	196,000
Accumulated depreciation			109,000	99,000
Depreciation expense			$ 10,500	9,800	8,000
Traditional life and health, interest rate, low end			2.50%
Traditional life and health, interest rate, high end			7.00%
Traditional life and health, weighted average interest rate			5.80%
Tax benefit as the result of a lower corporate tax rate			$ 877,000
Insurance policy charges			$ 17,000	18,300	$ 19,300
Term over which monthly data points are used to derive volatility			3 years
Reclassification from AOCI to retained earnings		$ 252,000
Retained earnings			$ 4,806,208	$ 3,890,798
Minimum | Equipment
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life			3 years
Minimum | Building and Improvements
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life			10 years
Maximum | Equipment
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life			5 years
Maximum | Building and Improvements
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life			40 years
Life insurance
Significant Accounting Policies [Line Items]
Proportion of future policy reserves which are not universal life type			87.00%
Employee | Equity Option | 5 Year Vesting Period | Torchmark Corporation 2011 Incentive Plan
Significant Accounting Policies [Line Items]
Option grants contractual term			10 years
Vesting period			5 years
Employee | Equity Option | 3 Year Vesting Period | Torchmark Corporation 2011 Incentive Plan
Significant Accounting Policies [Line Items]
Option grants contractual term			7 years
Vesting period			3 years
Director | Equity Option | Torchmark Corporation 2011 Incentive Plan
Significant Accounting Policies [Line Items]
Vesting period			6 months
Accounting Standards Update 2018-02
Significant Accounting Policies [Line Items]
Reclassification from AOCI to retained earnings	[1]		$ 252,000
Forecast | Accounting Standards Update 2017-07 | Minimum
Significant Accounting Policies [Line Items]
Impact of ASU on financial statements						$ 3,000
Forecast | Accounting Standards Update 2017-07 | Maximum
Significant Accounting Policies [Line Items]
Impact of ASU on financial statements						$ 5,000
Subsequent Event | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Retained earnings							$ 6,000

[1] Income tax effects of certain items were reclassified from accumulated other comprehensive income to retained earnings to remove stranded tax effects as a result of early adoption of ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. See further discussion in Note 1—Significant Accounting Policies.