Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue:
Premium	$ 929,835	$ 890,973
Net investment income	228,991	226,673
Other income	325	241
Total revenue	1,159,151	1,117,887
Expenses:
Policy benefits	607,969	587,757
Required interest on reserves	0	0
Required interest on DAC	0	0
Amortization of acquisition costs	143,837	135,822
Commissions, premium taxes, and non-deferred acquisition costs	78,937	73,465
Insurance administrative expense	66,895	59,591
Parent expense	2,331	2,643
Stock-based compensation expense	9,356	10,559
Interest expense	20,808	21,278
Total benefits and expenses	930,133	891,115
Subtotal	229,018	226,772
Non-operating items	3,275	400
Measure of segment profitability (pretax)	232,293	227,172
Realized gains (losses)	(26,097)	1,329
Legal proceedings	(3,275)
Administrative settlements		(400)
Income before income taxes	202,921	228,101
Operating Segments | Life
Revenue:
Premium	649,630	624,289
Net investment income	0	0
Other income	0	0
Total revenue	649,630	624,289
Expenses:
Policy benefits	421,670	409,692
Required interest on reserves	(171,205)	(163,662)
Required interest on DAC	52,118	50,024
Amortization of acquisition costs	114,308	108,290
Commissions, premium taxes, and non-deferred acquisition costs	53,936	50,106
Insurance administrative expense	0	0
Parent expense	0	0
Stock-based compensation expense	0	0
Interest expense	0	0
Total benefits and expenses	470,827	454,450
Subtotal	178,803	169,839
Measure of segment profitability (pretax)	178,803	169,839
Operating Segments | Health
Revenue:
Premium	280,205	266,684
Net investment income	0	0
Other income	0	0
Total revenue	280,205	266,684
Expenses:
Policy benefits	178,711	170,017
Required interest on reserves	(22,510)	(21,496)
Required interest on DAC	6,519	6,283
Amortization of acquisition costs	29,025	27,014
Commissions, premium taxes, and non-deferred acquisition costs	24,995	23,352
Insurance administrative expense	0	0
Parent expense	0	0
Stock-based compensation expense	0	0
Interest expense	0	0
Total benefits and expenses	216,740	205,170
Subtotal	63,465	61,514
Measure of segment profitability (pretax)	63,465	61,514
Operating Segments | Annuity
Revenue:
Premium	0	0
Net investment income	0	0
Other income	0	0
Total revenue	0	0
Expenses:
Policy benefits	7,588	8,048
Required interest on reserves	(10,456)	(11,120)
Required interest on DAC	88	131
Amortization of acquisition costs	504	518
Commissions, premium taxes, and non-deferred acquisition costs	6	7
Insurance administrative expense	0	0
Parent expense	0	0
Stock-based compensation expense	0	0
Interest expense	0	0
Total benefits and expenses	(2,270)	(2,416)
Subtotal	2,270	2,416
Measure of segment profitability (pretax)	2,270	2,416
Operating Segments | Investment
Revenue:
Premium	0	0
Net investment income	228,991	226,673
Other income	0	0
Total revenue	228,991	226,673
Expenses:
Policy benefits	0	0
Required interest on reserves	204,171	196,278
Required interest on DAC	(58,725)	(56,438)
Amortization of acquisition costs	0	0
Commissions, premium taxes, and non-deferred acquisition costs	0	0
Insurance administrative expense	0	0
Parent expense	0	0
Stock-based compensation expense	0	0
Interest expense	20,808	21,278
Total benefits and expenses	166,254	161,118
Subtotal	62,737	65,555
Measure of segment profitability (pretax)	62,737	65,555
Corporate & Other
Revenue:
Premium	0	0
Net investment income	0	0
Other income	325	241
Total revenue	325	241
Expenses:
Policy benefits	0	0
Required interest on reserves	0	0
Required interest on DAC	0	0
Amortization of acquisition costs	0	0
Commissions, premium taxes, and non-deferred acquisition costs	0	0
Insurance administrative expense	63,620	59,191
Parent expense	2,331	2,643
Stock-based compensation expense	9,356	10,559
Interest expense	0	0
Total benefits and expenses	75,307	72,393
Subtotal	(74,982)	(72,152)
Measure of segment profitability (pretax)	(74,982)	(72,152)
Adjustments
Revenue:
Other income		0
Total revenue	0	0
Expenses:
Commissions, premium taxes, and non-deferred acquisition costs		0
Insurance administrative expense	3,275	400
Parent expense	0	0
Total benefits and expenses	3,275	400
Subtotal	(3,275)	(400)
Non-operating items	3,275	$ 400
Measure of segment profitability (pretax)	$ 0