Business Segments - Schedule of Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue:
Premium	$ 1,130,672	$ 1,119,335	$ 1,110,920	$ 1,095,090	$ 1,088,949	$ 1,079,282	$ 1,077,199	$ 1,064,812	$ 4,456,017	$ 4,310,242	$ 4,094,813
Net investment income	271,609	266,926	261,244	257,105	255,483	246,711	244,712	244,894	1,056,884	991,800	956,690
Other income									308	1,246	1,216
Total revenue									5,513,209	5,303,288	5,052,719
Expenses:
Policy benefits	719,770	719,044	717,510	707,927	702,278	737,576	691,431	694,149	2,864,251	2,825,434	2,659,046
Required interest on reserves									0	0	0
Required interest on DAC									0		0
Amortization of acquisition costs	97,541	95,757	94,080	92,322	90,131	88,012	86,185	84,496	379,700	348,824	317,616
Commissions, premium taxes, and non-deferred acquisition costs									559,167	506,022	455,250
Insurance administrative expense									302,061	307,516	282,029
Parent expense									15,036	10,788	9,728
Stock-based compensation expense									30,736	35,650	30,272
Interest expense									102,316	90,395	83,486
Total benefits and expenses									4,253,267	4,124,629	3,837,427
Subtotal									1,259,942	1,178,659	1,215,292
Non-operating items									5,070	7,807	11,898
Measure of segment profitability (pretax)									1,265,012	1,186,466	1,227,190
Realized gain (loss)—investments											68,633
Realized gain (loss)—investments	$ 13,287	$ (2,193)	$ (45,843)	$ (30,927)	$ (9,703)	$ (29,155)	$ (30,446)	$ (7,244)	(65,676)	(76,548)	59,319
Realized loss—redemption of debt									0	0	(9,314)
Administrative settlements											(1,325)
Legal proceedings									(900)	(2,496)	(8,139)
Non-operating expenses									(4,170)	(5,311)	(2,434)
Income before income taxes									1,194,266	1,102,111	1,274,611
Life
Revenue:
Premium									3,137,244	3,027,824	2,893,930
Expenses:
Amortization of acquisition costs									327,426	298,841	270,928
Health
Revenue:
Premium									1,318,773	1,282,417	1,200,882
Expenses:
Amortization of acquisition costs									50,598	48,184	44,824
Annuity
Revenue:
Premium									0	1	1
Operating Segments
Expenses:
Non-operating items									0	0
Operating Segments | Life
Revenue:
Premium									3,137,244	3,027,824	2,893,930
Net investment income									0	0	0
Other income									0	0	0
Total revenue									3,137,244	3,027,824	2,893,930
Expenses:
Policy benefits									2,050,789	2,035,693	1,897,194
Required interest on reserves									(772,701)	(735,688)	(710,301)
Required interest on DAC									0		0
Amortization of acquisition costs									327,426	298,841	270,924
Commissions, premium taxes, and non-deferred acquisition costs									338,758	299,453	274,475
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total benefits and expenses									1,944,272	1,898,299	1,732,292
Subtotal									1,192,972	1,129,525	1,161,638
Non-operating items											0
Measure of segment profitability (pretax)									1,192,972	1,129,525	1,161,638
Operating Segments | Health
Revenue:
Premium									1,318,773	1,282,417	1,200,882
Net investment income									0	0	0
Other income									0	0	0
Total revenue									1,318,773	1,282,417	1,200,882
Expenses:
Policy benefits									776,362	752,866	721,309
Required interest on reserves									(106,516)	(102,315)	(98,477)
Required interest on DAC									0		0
Amortization of acquisition costs									50,598	48,185	44,824
Commissions, premium taxes, and non-deferred acquisition costs									220,392	206,544	180,748
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total benefits and expenses									940,836	905,280	848,404
Subtotal									377,937	377,137	352,478
Non-operating items											0
Measure of segment profitability (pretax)									377,937	377,137	352,478
Operating Segments | Annuity
Revenue:
Premium									0	1	1
Net investment income									0	0	0
Other income									0	0	0
Total revenue									0	1	1
Expenses:
Policy benefits									28,039	32,503	34,975
Required interest on reserves									(38,224)	(44,836)	(46,695)
Required interest on DAC									0		0
Amortization of acquisition costs									1,676	1,798	1,868
Commissions, premium taxes, and non-deferred acquisition costs									17	25	27
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total benefits and expenses									(8,492)	(10,510)	(9,825)
Subtotal									8,492	10,511	9,826
Non-operating items											0
Measure of segment profitability (pretax)									8,492	10,511	9,826
Operating Segments | Investment
Revenue:
Premium									0	0	0
Net investment income									1,056,884	991,800	956,690
Other income									0	0	0
Total revenue									1,056,884	991,800	956,690
Expenses:
Policy benefits									9,061	4,372	4,243
Required interest on reserves									917,441	882,839	855,473
Required interest on DAC									0		0
Amortization of acquisition costs									0	0	0
Commissions, premium taxes, and non-deferred acquisition costs									0	0	0
Insurance administrative expense									0	0	0
Parent expense									0	0	0
Stock-based compensation expense									0	0	0
Interest expense									0	0
Total benefits and expenses									926,502	887,211	859,716
Subtotal									130,382	104,589	96,974
Non-operating items											0
Measure of segment profitability (pretax)									130,382	104,589	96,974
Corporate & Other
Revenue:
Premium									0	0	0
Net investment income									0	0	0
Other income									308	1,246	1,216
Total revenue									308	1,246	1,216
Expenses:
Policy benefits									0	0	0
Required interest on reserves									0	0	0
Required interest on DAC									0		0
Amortization of acquisition costs									0	0	0
Commissions, premium taxes, and non-deferred acquisition costs									0	0	0
Insurance administrative expense									301,161	299,341	271,631
Parent expense									10,866	11,156	9,553
Stock-based compensation expense									30,736	35,650	30,272
Interest expense									102,316	90,395	83,486
Total benefits and expenses									445,079	436,542	394,942
Subtotal									(444,771)	(435,296)	(393,726)
Non-operating items									0	0	0
Measure of segment profitability (pretax)									(444,771)	(435,296)	(393,726)
Adjustments
Revenue:
Premium									0	0	0
Net investment income									0	0	0
Other income									0	0	0
Total revenue									0	0	0
Expenses:
Policy benefits									0	0	1,325
Required interest on reserves									0	0	0
Required interest on DAC									0		0
Amortization of acquisition costs									0	0	0
Commissions, premium taxes, and non-deferred acquisition costs									0	0	0
Insurance administrative expense									900	8,175	10,398
Parent expense									4,170	(368)	175
Stock-based compensation expense									0	0	0
Interest expense									0	0	0
Total benefits and expenses									5,070	7,807	11,898
Subtotal									(5,070)	(7,807)	(11,898)
Non-operating items									5,070	7,807	11,898
Measure of segment profitability (pretax)									$ 0	$ 0	$ 0