Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
Premium	$ 826,473	$ 819,217	$ 816,614	$ 820,631	$ 787,908	$ 783,411	$ 785,855	$ 779,860	$ 3,282,935	$ 3,137,034	$ 2,998,720
Net investment income	212,955	213,872	212,776	208,282	205,488	202,720	201,642	197,053	847,885	806,903	773,951
Other income									1,142	1,375	2,185
Total revenue									4,131,962	3,945,312	3,774,856
Expenses:
Policy benefits	562,465	551,219	556,415	557,776	540,138	532,152	531,485	524,973	2,227,875	2,128,748	2,016,212
Policy reserves									0	0	0
Deferred acquisition costs									0	0	0
Amortization of acquisition costs	120,040	122,334	122,121	125,908	116,191	116,821	117,245	118,806	490,403	469,063	445,625
Commissions, premium taxes, and non-deferred acquisition costs									264,860	249,174	237,541
Insurance administrative expense									210,590	197,151	186,191
Parent expense									9,631	8,587	9,003
Stock-based compensation expense									37,034	26,326	28,664
Stock-based compensation expense									3,380
Interest expense									84,532	83,345	76,642
Total benefits and expenses									3,324,925	3,162,394	2,999,878
Subtotal									807,037	782,918	774,978
Non-operating items									14,128	4,348	1,394
Measure of segment profitability (pretax)									821,165	787,266	776,372
Deduct applicable income taxes									(247,484)	(237,906)	(253,459)
Net operating income from continuing operations									573,681	549,360	522,913
Add (deduct) realized investment gains (losses)	17,469	12,595	(705)	(5,748)	(18,463)	3,482	4,005	293	23,611	(10,683)	(8,791)
Deduct guaranty fund assessments									(1,801)
Deduct increase in stock-based compensation expense due to Tax Legislation									(3,380)
Deduct administrative settlements									(8,659)	(3,795)	(1,394)
Deduct non-operating fees									(288)	(553)
Income before income taxes per Consolidated Statement of Operations	$ 216,371	$ 220,610	$ 201,926	$ 191,741	$ 175,982	$ 201,461	$ 199,344	$ 195,448	830,648	772,235	766,187
Operating Segments | Life
Revenue:
Premium									2,306,547	2,189,333	2,073,065
Total revenue									2,306,547	2,189,333	2,073,065
Expenses:
Policy benefits									1,549,602	1,475,477	1,374,608
Policy reserves									(607,007)	(577,827)	(552,298)
Deferred acquisition costs									186,236	178,946	172,947
Amortization of acquisition costs									396,268	374,499	353,595
Commissions, premium taxes, and non-deferred acquisition costs									177,111	164,476	154,811
Total benefits and expenses									1,702,210	1,615,571	1,503,663
Subtotal									604,337	573,762	569,402
Measure of segment profitability (pretax)									604,337	573,762	569,402
Operating Segments | Health
Revenue:
Premium									976,373	947,663	925,520
Total revenue									976,373	947,663	925,520
Expenses:
Policy benefits									628,640	612,725	602,610
Policy reserves									(77,792)	(73,382)	(69,057)
Deferred acquisition costs									23,454	23,060	22,760
Amortization of acquisition costs									96,519	90,385	83,341
Commissions, premium taxes, and non-deferred acquisition costs									86,044	84,819	81,489
Total benefits and expenses									756,865	737,607	721,143
Subtotal									219,508	210,056	204,377
Measure of segment profitability (pretax)									219,508	210,056	204,377
Operating Segments | Annuity
Revenue:
Premium									15	38	135
Total revenue									15	38	135
Expenses:
Policy benefits									35,836	36,751	38,994
Policy reserves									(49,571)	(51,131)	(53,295)
Deferred acquisition costs									690	807	1,138
Amortization of acquisition costs									2,466	4,179	8,689
Commissions, premium taxes, and non-deferred acquisition costs									32	38	41
Total benefits and expenses									(10,547)	(9,356)	(4,433)
Subtotal									10,562	9,394	4,568
Measure of segment profitability (pretax)									10,562	9,394	4,568
Operating Segments | Investment
Revenue:
Net investment income									847,885	806,903	773,951
Total revenue									847,885	806,903	773,951
Expenses:
Policy reserves									734,370	702,340	674,650
Deferred acquisition costs									(210,380)	(202,813)	(196,845)
Interest expense									84,532	83,345	76,642
Total benefits and expenses									608,522	582,872	554,447
Subtotal									239,363	224,031	219,504
Measure of segment profitability (pretax)									239,363	224,031	219,504
Operating Segments | Corporate & Other
Revenue:
Other income									1,270	1,534	2,379
Total revenue									1,270	1,534	2,379
Expenses:
Insurance administrative expense									210,590	196,598	186,191
Parent expense									9,631	8,587	9,003
Stock-based compensation expense									33,654	26,326	28,664
Total benefits and expenses									253,875	231,511	223,858
Subtotal									(252,605)	(229,977)	(221,479)
Measure of segment profitability (pretax)									(252,605)	(229,977)	(221,479)
Adjustments
Revenue:
Other income									(128)	(159)	(194)
Total revenue									(128)	(159)	(194)
Expenses:
Policy benefits									13,797	3,795
Amortization of acquisition costs									(4,850)
Commissions, premium taxes, and non-deferred acquisition costs									1,673	(159)	1,200
Insurance administrative expense										553
Stock-based compensation expense									3,380
Total benefits and expenses									14,000	4,189	1,200
Subtotal									(14,128)	(4,348)	(1,394)
Non-operating items									14,128		1,394
Measure of segment profitability (pretax)									0	$ 0	$ 0
Deduct increase in stock-based compensation expense due to Tax Legislation									$ (3,380)