Business Segments - Schedule of Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Revenue:
Premium	$ 1,110,920	$ 1,077,199		$ 2,206,010	$ 2,142,011
Net investment income	261,244	244,712		518,349	489,606
Other income	85	299		135	463
Total revenue	1,372,249	1,322,210		2,724,494	2,632,080
Expenses:
Policy benefits	717,510	691,431		1,425,437	1,385,580
Required interest on reserves	0	0		0	0
Amortization of acquisition costs	94,080	86,185		186,402	170,681
Commissions, premium taxes, and non-deferred acquisition costs	138,459	126,213		276,256	251,722
Insurance administrative expense	75,459	78,685		149,366	151,362
Parent expense	3,088	2,525		5,673	5,165
Stock-based compensation expense	7,486	8,448		15,165	17,483
Interest expense	25,818	21,828		50,685	41,772
Total benefits and expenses	1,061,900	1,015,315		2,108,984	2,023,765
Subtotal	310,349	306,895		615,510	608,315
Non-operating items	0	4,617		0	4,729
Measure of segment profitability (pretax)	310,349	311,512		615,510	613,044
Realized gain (loss)—investments	(45,843)	(30,446)		(76,770)	(37,690)
Non-operating expenses		(4,617)			(4,729)
Income before income taxes	264,506	276,449		538,740	570,625
Life
Revenue:
Premium	781,733	756,892		1,554,330	1,506,020
Expenses:
Amortization of acquisition costs	81,173		$ 73,681	160,762	145,610
Health
Revenue:
Premium	329,187	320,307		651,680	635,991
Expenses:
Amortization of acquisition costs	12,484	12,052		24,792	24,166
Annuity
Revenue:
Premium	0	0		0	0
Operating Segments
Expenses:
Non-operating items	0	0		0	0
Operating Segments | Life
Revenue:
Premium	781,733	756,892		1,554,330	1,506,020
Net investment income	0	0		0	0
Other income	0	0		0	0
Total revenue	781,733	756,892		1,554,330	1,506,020
Expenses:
Policy benefits	512,664	492,364		1,020,641	987,793
Required interest on reserves	(191,781)	(182,756)		(381,602)	(364,128)
Amortization of acquisition costs	81,173	73,681		160,762	145,610
Commissions, premium taxes, and non-deferred acquisition costs	83,547	74,478		167,125	148,026
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	485,603	457,767		966,926	917,301
Subtotal	296,130	299,125		587,404	588,719
Measure of segment profitability (pretax)	296,130	299,125		587,404	588,719
Operating Segments | Health
Revenue:
Premium	329,187	320,307		651,680	635,991
Net investment income	0	0		0	0
Other income	0	0		0	0
Total revenue	329,187	320,307		651,680	635,991
Expenses:
Policy benefits	195,924	190,075		386,886	379,093
Required interest on reserves	(26,548)	(25,460)		(52,871)	(50,730)
Amortization of acquisition costs	12,484	12,052		24,792	24,166
Commissions, premium taxes, and non-deferred acquisition costs	54,908	51,729		109,122	103,681
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	236,768	228,396		467,929	456,210
Subtotal	92,419	91,911		183,751	179,781
Measure of segment profitability (pretax)	92,419	91,911		183,751	179,781
Operating Segments | Annuity
Revenue:
Premium	0	0		0	0
Net investment income	0	0		0	0
Other income	0	0		0	0
Total revenue	0	0		0	0
Expenses:
Policy benefits	7,107	7,922		14,648	16,564
Required interest on reserves	(9,715)	(11,361)		(19,974)	(22,728)
Amortization of acquisition costs	423	452		848	905
Commissions, premium taxes, and non-deferred acquisition costs	4	6		9	15
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	(2,181)	(2,981)		(4,469)	(5,244)
Subtotal	2,181	2,981		4,469	5,244
Measure of segment profitability (pretax)	2,181	2,981		4,469	5,244
Operating Segments | Investment
Revenue:
Premium	0	0		0	0
Net investment income	261,244	244,712		518,349	489,606
Other income	0	0		0	0
Total revenue	261,244	244,712		518,349	489,606
Expenses:
Policy benefits	1,815	1,070		3,262	2,130
Required interest on reserves	228,044	219,577		454,447	437,586
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	0	0		0	0
Total benefits and expenses	229,859	220,647		457,709	439,716
Subtotal	31,385	24,065		60,640	49,890
Measure of segment profitability (pretax)	31,385	24,065		60,640	49,890
Corporate & Other
Revenue:
Premium	0	0		0	0
Net investment income	0	0		0	0
Other income	85	299		135	463
Total revenue	85	299		135	463
Expenses:
Policy benefits	0	0		0	0
Required interest on reserves	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	75,459	73,700		149,366	146,265
Parent expense	3,088	2,893		5,673	5,533
Stock-based compensation expense	7,486	8,448		15,165	17,483
Interest expense	25,818	21,828		50,685	41,772
Total benefits and expenses	111,851	106,869		220,889	211,053
Subtotal	(111,766)	(106,570)		(220,754)	(210,590)
Non-operating items	0	0		0	0
Measure of segment profitability (pretax)	(111,766)	(106,570)		(220,754)	(210,590)
Adjustments
Revenue:
Premium	0	0		0	0
Net investment income	0	0		0	0
Other income	0	0		0	0
Total revenue	0	0		0	0
Expenses:
Policy benefits	0	0		0	0
Required interest on reserves	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		4,985			5,097
Parent expense	0	(368)			(368)
Stock-based compensation expense	0	0		0	0
Interest expense	0	0		0	0
Total benefits and expenses	0	4,617		0	4,729
Subtotal	0	(4,617)		0	(4,729)
Non-operating items		4,617		0	4,729
Measure of segment profitability (pretax)	$ 0	$ 0		$ 0	$ 0