Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue:
Premium	$ 899,993		$ 860,750	$ 2,688,450	$ 2,564,835
Net investment income	228,905		221,627	683,003	658,279
Other income	438		393	1,077	1,104
Total revenue	1,129,336		1,082,770	3,372,530	3,224,218
Expenses:
Policy benefits	585,692		567,856	1,762,811	1,706,122
Required interest on reserves	0		0	0	0
Required interest on DAC	0		0	0	0
Amortization of acquisition costs	138,449		129,492	412,436	388,189
Commissions, premium taxes, and non-deferred acquisition costs	74,139		69,632	221,302	208,698
Insurance administrative expense	60,570		59,402	185,077	170,150
Parent expense	2,472		4,245	7,987	9,384
Stock-based compensation expense	11,533		10,412	33,348	29,969
Interest expense	21,094		22,433	63,804	66,466
Total benefits and expenses	893,949		863,472	2,686,765	2,578,978
Subtotal	235,387		219,298	685,765	645,240
Non-operating items	0		5,168	5,900	5,168
Measure of segment profitability (pretax)	235,387		224,466	691,665	650,408
Realized gain (loss)—investments	11,943		1,032	18,426	14,796
Administrative settlements		$ (400)	(3,590)	(400)	(3,590)
Non-operating fees			(1,578)		(1,578)
Legal proceedings				(5,500)
Income before income taxes	247,330		220,330	704,191	660,036
Operating Segments | Life
Revenue:
Premium	630,824		605,547	1,886,314	1,806,384
Total revenue	630,824		605,547	1,886,314	1,806,384
Expenses:
Policy benefits	406,963		396,701	1,227,616	1,196,616
Required interest on reserves	(167,719)		(160,058)	(496,894)	(474,373)
Required interest on DAC	50,948		48,907	151,270	145,126
Amortization of acquisition costs	108,798		103,792	327,407	312,168
Commissions, premium taxes, and non-deferred acquisition costs	50,375		47,609	150,567	142,253
Total benefits and expenses	449,365		436,951	1,359,966	1,321,790
Subtotal	181,459		168,596	526,348	484,594
Measure of segment profitability (pretax)	181,459		168,596	526,348	484,594
Operating Segments | Health
Revenue:
Premium	269,166		255,201	802,132	758,439
Total revenue	269,166		255,201	802,132	758,439
Expenses:
Policy benefits	170,875		162,574	511,403	483,654
Required interest on reserves	(21,955)		(20,965)	(65,131)	(62,095)
Required interest on DAC	6,395		6,136	19,019	18,228
Amortization of acquisition costs	29,151		25,162	83,508	74,311
Commissions, premium taxes, and non-deferred acquisition costs	23,759		22,041	70,719	66,502
Total benefits and expenses	208,225		194,948	619,518	580,600
Subtotal	60,941		60,253	182,614	177,839
Measure of segment profitability (pretax)	60,941		60,253	182,614	177,839
Operating Segments | Annuity
Revenue:
Premium	3		2	4	12
Total revenue	3		2	4	12
Expenses:
Policy benefits	7,854		8,581	23,792	25,852
Required interest on reserves	(10,832)		(11,831)	(32,840)	(35,733)
Required interest on DAC	125		145	383	448
Amortization of acquisition costs	500		538	1,521	1,710
Commissions, premium taxes, and non-deferred acquisition costs	5		5	16	19
Total benefits and expenses	(2,348)		(2,562)	(7,128)	(7,704)
Subtotal	2,351		2,564	7,132	7,716
Measure of segment profitability (pretax)	2,351		2,564	7,132	7,716
Operating Segments | Investment
Revenue:
Premium					0
Net investment income	228,905		221,627	683,003	658,279
Total revenue	228,905		221,627	683,003	658,279
Expenses:
Required interest on reserves	200,506		192,854	594,865	572,201
Required interest on DAC	(57,468)		(55,188)	(170,672)	(163,802)
Interest expense	21,094		22,433	63,804	66,466
Total benefits and expenses	164,132		160,099	487,997	474,865
Subtotal	64,773		61,528	195,006	183,414
Measure of segment profitability (pretax)	64,773		61,528	195,006	183,414
Operating Segments | Corporate & Other
Revenue:
Premium					0
Other income	438		416	1,077	1,180
Total revenue	438		416	1,077	1,180
Expenses:
Insurance administrative expense	60,570		55,812	179,177	166,560
Parent expense	2,472		2,667	7,987	7,806
Stock-based compensation expense	11,533		10,412	33,348	29,969
Total benefits and expenses	74,575		68,891	220,512	204,335
Subtotal	(74,137)		(68,475)	(219,435)	(203,155)
Measure of segment profitability (pretax)	(74,137)		(68,475)	(219,435)	(203,155)
Adjustments
Revenue:
Premium					0
Net investment income					0
Other income	0		(23)	0	(76)
Total revenue	0		(23)	0	(76)
Expenses:
Policy benefits	0			0
Commissions, premium taxes, and non-deferred acquisition costs	0		(23)	0	(76)
Insurance administrative expense	0		3,590	5,900	3,590
Parent expense			1,578		1,578
Total benefits and expenses	0		5,145	5,900	5,092
Subtotal	0		(5,168)	(5,900)	(5,168)
Non-operating items	0		5,168	5,900	5,168
Measure of segment profitability (pretax)	$ 0		$ 0	$ 0	$ 0