Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Revenue:
Premium	$ 953,702	$ 897,484	$ 1,883,537	$ 1,788,457
Net investment income	231,568	227,425	460,559	454,098
Other income	404	398	729	639
Total revenue	1,185,674	1,125,307	2,344,825	2,243,194
Expenses:
Policy benefits	650,816	589,362	1,258,785	1,177,119
Required interest on reserves	0	0	0	0
Required interest on DAC	0	0	0	0
Amortization of acquisition costs	146,160	138,165	289,997	273,987
Commissions, premium taxes, and non-deferred acquisition costs	76,140	73,698	155,077	147,163
Insurance administrative expense	61,566	64,916	128,461	124,507
Parent expense	2,516	2,872	4,847	5,515
Stock-based compensation expense	8,632	11,256	17,988	21,815
Interest expense	22,813	21,432	43,621	42,710
Total benefits and expenses	968,643	901,701	1,898,776	1,792,816
Subtotal	217,031	223,606	446,049	450,378
Non-operating items	0	5,500	3,275	5,900
Measure of segment profitability (pretax)	217,031	229,106	449,324	456,278
Realized gains (losses)	(4,790)	5,154	(30,887)	6,483
Legal proceedings			(3,275)	(5,500)	$ 5,500
Administrative settlements				(400)
Income before income taxes	212,241	228,760	415,162	456,861
Operating Segments | Life
Revenue:
Premium	670,822	631,201	1,320,452	1,255,490
Net investment income	0	0	0	0
Other income	0	0	0	0
Total revenue	670,822	631,201	1,320,452	1,255,490
Expenses:
Policy benefits	459,845	410,961	881,515	820,653
Required interest on reserves	(173,208)	(165,513)	(344,413)	(329,175)
Required interest on DAC	52,107	50,298	104,225	100,322
Amortization of acquisition costs	117,802	110,319	232,110	218,609
Commissions, premium taxes, and non-deferred acquisition costs	52,577	50,086	106,513	100,192
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	509,123	456,151	979,950	910,601
Subtotal	161,699	175,050	340,502	344,889
Measure of segment profitability (pretax)	161,699	175,050	340,502	344,889
Operating Segments | Health
Revenue:
Premium	282,877	266,282	563,082	532,966
Net investment income	0	0	0	0
Other income	0	0	0	0
Total revenue	282,877	266,282	563,082	532,966
Expenses:
Policy benefits	183,496	170,511	362,207	340,528
Required interest on reserves	(22,851)	(21,680)	(45,361)	(43,176)
Required interest on DAC	6,582	6,341	13,101	12,624
Amortization of acquisition costs	27,854	27,343	56,879	54,357
Commissions, premium taxes, and non-deferred acquisition costs	23,556	23,608	48,551	46,960
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	218,637	206,123	435,377	411,293
Subtotal	64,240	60,159	127,705	121,673
Measure of segment profitability (pretax)	64,240	60,159	127,705	121,673
Operating Segments | Annuity
Revenue:
Premium	3	1	3	1
Net investment income	0	0	0	0
Other income	0	0	0	0
Total revenue	3	1	3	1
Expenses:
Policy benefits	7,475	7,890	15,063	15,938
Required interest on reserves	(10,332)	(10,888)	(20,788)	(22,008)
Required interest on DAC	84	127	172	258
Amortization of acquisition costs	504	503	1,008	1,021
Commissions, premium taxes, and non-deferred acquisition costs	7	4	13	11
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	(2,262)	(2,364)	(4,532)	(4,780)
Subtotal	2,265	2,365	4,535	4,781
Measure of segment profitability (pretax)	2,265	2,365	4,535	4,781
Operating Segments | Investment
Revenue:
Premium	0	0	0	0
Net investment income	231,568	227,425	460,559	454,098
Other income	0	0	0	0
Total revenue	231,568	227,425	460,559	454,098
Expenses:
Policy benefits	0	0	0	0
Required interest on reserves	206,391	198,081	410,562	394,359
Required interest on DAC	(58,773)	(56,766)	(117,498)	(113,204)
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	22,813	21,432	43,621	42,710
Total benefits and expenses	170,431	162,747	336,685	323,865
Subtotal	61,137	64,678	123,874	130,233
Measure of segment profitability (pretax)	61,137	64,678	123,874	130,233
Corporate & Other
Revenue:
Premium	0	0	0	0
Net investment income	0	0	0	0
Other income	404	398	729	639
Total revenue	404	398	729	639
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	61,566	59,416	125,186	118,607
Parent expense	2,516	2,872	4,847	5,515
Stock-based compensation expense	8,632	11,256	17,988	21,815
Interest expense	0	0	0	0
Total benefits and expenses	72,714	73,544	148,021	145,937
Subtotal	(72,310)	(73,146)	(147,292)	(145,298)
Measure of segment profitability (pretax)	(72,310)	(73,146)	(147,292)	(145,298)
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		5,500	3,275	5,900
Parent expense	0	0	0	0
Total benefits and expenses	0	5,500	3,275	5,900
Subtotal	0	(5,500)	(3,275)	(5,900)
Non-operating items		5,500	3,275	$ 5,900
Measure of segment profitability (pretax)	$ 0	$ 0	$ 0