Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue:
Premium	$ 961,817	$ 899,993	$ 2,845,354	$ 2,688,450
Net investment income	231,432	228,905	691,991	683,003
Other income	292	438	1,021	1,077
Total revenue	1,193,541	1,129,336	3,538,366	3,372,530
Expenses:
Policy benefits	650,976	585,692	1,909,761	1,762,811
Required interest on reserves	0	0	0	0
Required interest on DAC	0	0	0	0
Amortization of acquisition costs	140,843	138,449	430,840	412,436
Commissions, premium taxes, and non-deferred acquisition costs	74,614	74,139	229,691	221,302
Insurance administrative expense	63,718	60,570	192,179	185,077
Parent expense	3,012	2,472	7,859	7,987
Stock-based compensation expense	8,667	11,533	26,655	33,348
Interest expense	21,674	21,094	65,295	63,804
Total benefits and expenses	963,504	893,949	2,862,280	2,686,765
Subtotal	230,037	235,387	676,086	685,765
Non-operating items	1,033	0	4,308	5,900
Measure of segment profitability (pretax)	231,070	235,387	680,394	691,665
Realized gains (losses)	2,135	11,943	(28,752)	18,426
Administrative settlements				(400)
Legal proceedings			(3,275)	(5,500)
Income before income taxes	231,538	247,330	646,700	704,191
Premium Receivable, Credit Loss Expense (Reversal)	(634)		(634)
Non-operating fees	(1,033)		(1,033)
Operating Segments | Life
Revenue:
Premium	674,021	630,824	1,994,473	1,886,314
Net investment income	0	0	0	0
Other income	0	0	0	0
Total revenue	674,021	630,824	1,994,473	1,886,314
Expenses:
Policy benefits	459,231	406,963	1,340,746	1,227,616
Required interest on reserves	(175,794)	(167,719)	(520,207)	(496,894)
Required interest on DAC	52,709	50,948	156,934	151,270
Amortization of acquisition costs	114,316	108,798	346,426	327,407
Commissions, premium taxes, and non-deferred acquisition costs	52,856	50,375	159,369	150,567
Insurance administrative expense	0	0	0	0
Parent expense	0	0	0	0
Stock-based compensation expense	0	0	0	0
Interest expense	0	0	0	0
Total benefits and expenses	503,318	449,365	1,483,268	1,359,966
Subtotal	170,703	181,459	511,205	526,348
Measure of segment profitability (pretax)	170,703	181,459	511,205	526,348
Operating Segments | Health
Revenue:
Premium	287,795	269,166	850,877	802,132
Net investment income	0	0	0	0
Other income	0	0	0	0
Total revenue	287,795	269,166	850,877	802,132
Expenses:
Policy benefits	184,237	170,875	546,444	511,403
Required interest on reserves	(23,770)	(21,955)	(69,131)	(65,131)
Required interest on DAC	6,689	6,395	19,790	19,019
Amortization of acquisition costs	26,026	29,151	82,905	83,508
Commissions, premium taxes, and non-deferred acquisition costs	21,753	23,759	70,304	70,719
Insurance administrative expense	0	0	0	0
Parent expense	0	0	0	0
Stock-based compensation expense	0	0	0	0
Interest expense	0	0	0	0
Total benefits and expenses	214,935	208,225	650,312	619,518
Subtotal	72,860	60,941	200,565	182,614
Measure of segment profitability (pretax)	72,860	60,941	200,565	182,614
Operating Segments | Annuity
Revenue:
Premium	1	3	4	4
Net investment income	0	0	0	0
Other income	0	0	0	0
Total revenue	1	3	4	4
Expenses:
Policy benefits	7,508	7,854	22,571	23,792
Required interest on reserves	(10,347)	(10,832)	(31,135)	(32,840)
Required interest on DAC	81	125	253	383
Amortization of acquisition costs	501	500	1,509	1,521
Commissions, premium taxes, and non-deferred acquisition costs	5	5	18	16
Insurance administrative expense	0	0	0	0
Parent expense	0	0	0	0
Stock-based compensation expense	0	0	0	0
Interest expense	0	0	0	0
Total benefits and expenses	(2,252)	(2,348)	(6,784)	(7,128)
Subtotal	2,253	2,351	6,788	7,132
Measure of segment profitability (pretax)	2,253	2,351	6,788	7,132
Operating Segments | Investment
Revenue:
Premium	0	0	0	0
Net investment income	231,432	228,905	691,991	683,003
Other income	0	0	0	0
Total revenue	231,432	228,905	691,991	683,003
Expenses:
Policy benefits	0	0	0	0
Required interest on reserves	209,911	200,506	620,473	594,865
Required interest on DAC	(59,479)	(57,468)	(176,977)	(170,672)
Amortization of acquisition costs	0	0	0	0
Commissions, premium taxes, and non-deferred acquisition costs	0	0	0	0
Insurance administrative expense	0	0	0	0
Parent expense	0	0	0	0
Stock-based compensation expense	0	0	0	0
Interest expense	21,674	21,094	65,295	63,804
Total benefits and expenses	172,106	164,132	508,791	487,997
Subtotal	59,326	64,773	183,200	195,006
Measure of segment profitability (pretax)	59,326	64,773	183,200	195,006
Corporate & Other
Revenue:
Premium	0	0	0	0
Net investment income	0	0	0	0
Other income	292	438	1,021	1,077
Total revenue	292	438	1,021	1,077
Expenses:
Policy benefits	0	0	0	0
Required interest on reserves	0	0	0	0
Required interest on DAC	0	0	0	0
Amortization of acquisition costs	0	0	0	0
Commissions, premium taxes, and non-deferred acquisition costs	0	0	0	0
Insurance administrative expense	63,008	60,570	188,194	179,177
Parent expense	2,689	2,472	7,536	7,987
Stock-based compensation expense	8,667	11,533	26,655	33,348
Interest expense	0	0	0	0
Total benefits and expenses	74,364	74,575	222,385	220,512
Subtotal	(74,072)	(74,137)	(221,364)	(219,435)
Measure of segment profitability (pretax)	(74,072)	(74,137)	(221,364)	(219,435)
Adjustments
Revenue:
Other income	0	0		0
Total revenue	0	0	0	0
Expenses:
Policy benefits	0
Commissions, premium taxes, and non-deferred acquisition costs	0	0		0
Insurance administrative expense	710	0	3,985	5,900
Parent expense	323	0	323	0
Total benefits and expenses	1,033	0	4,308	5,900
Subtotal	(1,033)	0	(4,308)	(5,900)
Non-operating items	1,033	0	4,308	$ 5,900
Measure of segment profitability (pretax)	$ 0	$ 0	$ 0