Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue:
Premium	$ 897,484		$ 853,979	$ 1,788,457	$ 1,704,085
Net investment income	227,425		218,568	454,098	436,652
Other income	398		416	639	711
Total revenue	1,125,307		1,072,963	2,243,194	2,141,448
Expenses:
Policy benefits	589,362		568,377	1,177,119	1,138,266
Required interest on reserves	0		0	0	0
Required interest on DAC	0		0	0	0
Amortization of acquisition costs	138,165		129,077	273,987	258,697
Commissions, premium taxes, and non-deferred acquisition costs	73,698		69,427	147,163	139,066
Insurance administrative expense	64,916		55,276	124,507	110,748
Parent expense	2,872		2,847	5,515	5,139
Stock-based compensation expense	11,256		10,497	21,815	19,557
Interest expense	21,432		22,411	42,710	44,033
Total benefits and expenses	901,701		857,912	1,792,816	1,715,506
Subtotal	223,606		215,051	450,378	425,942
Non-operating items	5,500		0	5,900	0
Measure of segment profitability (pretax)	229,106		215,051	456,278	425,942
Realized gain (loss)—investments	5,154		11,813	6,483	13,764
Administrative settlements		$ (400)		(400)
Legal proceedings	(5,500)			(5,500)
Income before income taxes	228,760		226,864	456,861	439,706
Operating Segments | Life
Revenue:
Premium	631,201		602,534	1,255,490	1,200,837
Total revenue	631,201		602,534	1,255,490	1,200,837
Expenses:
Policy benefits	410,961		399,334	820,653	799,915
Required interest on reserves	(165,513)		(158,101)	(329,175)	(314,315)
Required interest on DAC	50,298		48,275	100,322	96,219
Amortization of acquisition costs	110,319		104,599	218,609	208,376
Commissions, premium taxes, and non-deferred acquisition costs	50,086		47,250	100,192	94,644
Total benefits and expenses	456,151		441,357	910,601	884,839
Subtotal	175,050		161,177	344,889	315,998
Measure of segment profitability (pretax)	175,050		161,177	344,889	315,998
Operating Segments | Health
Revenue:
Premium	266,282		251,440	532,966	503,238
Total revenue	266,282		251,440	532,966	503,238
Expenses:
Policy benefits	170,511		160,461	340,528	321,080
Required interest on reserves	(21,680)		(20,726)	(43,176)	(41,130)
Required interest on DAC	6,341		6,079	12,624	12,092
Amortization of acquisition costs	27,343		23,892	54,357	49,149
Commissions, premium taxes, and non-deferred acquisition costs	23,608		22,196	46,960	44,461
Total benefits and expenses	206,123		191,902	411,293	385,652
Subtotal	60,159		59,538	121,673	117,586
Measure of segment profitability (pretax)	60,159		59,538	121,673	117,586
Operating Segments | Annuity
Revenue:
Premium	1		5	1	10
Total revenue	1		5	1	10
Expenses:
Policy benefits	7,890		8,582	15,938	17,271
Required interest on reserves	(10,888)		(11,885)	(22,008)	(23,902)
Required interest on DAC	127		150	258	303
Amortization of acquisition costs	503		586	1,021	1,172
Commissions, premium taxes, and non-deferred acquisition costs	4		6	11	14
Total benefits and expenses	(2,364)		(2,561)	(4,780)	(5,142)
Subtotal	2,365		2,566	4,781	5,152
Measure of segment profitability (pretax)	2,365		2,566	4,781	5,152
Operating Segments | Investment
Revenue:
Net investment income	227,425		218,568	454,098	436,652
Total revenue	227,425		218,568	454,098	436,652
Expenses:
Required interest on reserves	198,081		190,712	394,359	379,347
Required interest on DAC	(56,766)		(54,504)	(113,204)	(108,614)
Interest expense	21,432		22,411	42,710	44,033
Total benefits and expenses	162,747		158,619	323,865	314,766
Subtotal	64,678		59,949	130,233	121,886
Measure of segment profitability (pretax)	64,678		59,949	130,233	121,886
Operating Segments | Corporate & Other
Revenue:
Other income	398		441	639	764
Total revenue	398		441	639	764
Expenses:
Insurance administrative expense	59,416		55,276	118,607	110,748
Parent expense	2,872		2,847	5,515	5,139
Stock-based compensation expense	11,256		10,497	21,815	19,557
Total benefits and expenses	73,544		68,620	145,937	135,444
Subtotal	(73,146)		(68,179)	(145,298)	(134,680)
Measure of segment profitability (pretax)	(73,146)		(68,179)	(145,298)	(134,680)
Adjustments
Revenue:
Net investment income					0
Other income	0		(25)	0	(53)
Total revenue	0		(25)	0	(53)
Expenses:
Policy benefits	0			0
Amortization of acquisition costs					0
Commissions, premium taxes, and non-deferred acquisition costs	0		(25)	0	(53)
Insurance administrative expense	5,500			5,900
Interest expense					0
Total benefits and expenses	5,500		(25)	5,900	(53)
Subtotal	(5,500)		0	(5,900)	0
Non-operating items	5,500		0	5,900	0
Measure of segment profitability (pretax)	$ 0		$ 0	$ 0	$ 0