Policyholder Account Balances, Future Policy Benefits and Claims - Balances of and Changes in Liability for Future Policy Benefits (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Future Policy Benefit, Activity [Line Items]
Balance	$ 9,233	$ 9,378
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	1,953	2,027
Beginning balance at original discount rate	1,950	2,065
Effect of changes in cash flow assumptions		0	$ (19)
Effect of actual variances from expected experience		(9)	(23)
Adjusted beginning of year balance		1,941	2,023
Issuances	78	244
Interest accrual	24	101
Net premiums collected	(117)	(418)
Derecognition (lapses)	0	0
Ending balance at original discount rate	1,926	1,950
Effect of changes in discount rate assumptions	(27)	3
Ending balance	1,899	1,953
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	9,711	9,519
Beginning balance at original discount rate	9,641	9,706
Effect of changes in cash flow assumptions		0	(34)
Effect of actual variances from expected experience		(16)	(35)
Adjusted beginning of year balance		9,625	9,637
Issuances	78	244
Interest accrual	122	489
Benefit payments	(187)	(729)
Derecognition (lapses)	0	0
Ending balance at original discount rate	9,638	9,641
Effect of changes in discount rate assumptions	(126)	70
Ending balance	9,512	9,711
Adjustment due to reserve flooring	6	5
Reserve for future policy benefits	7,619	7,763
Less: reinsurance recoverable	3,969	4,080
Net liability for future policy benefits, after reinsurance recoverable	3,650	3,683
Discounted expected future gross premiums	3,871	3,993
Expected future gross premiums	5,899	5,993
Expected future benefit payments	15,744	15,810
Life Contingent Payout Annuities
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	0	0
Beginning balance at original discount rate	0	0
Effect of changes in cash flow assumptions		0	0
Effect of actual variances from expected experience		0	0
Adjusted beginning of year balance		0	0
Issuances	60	177
Interest accrual	0	1
Net premiums collected	(60)	(178)
Derecognition (lapses)	0	0
Ending balance at original discount rate	0	0
Effect of changes in discount rate assumptions	0	0
Ending balance	0	0
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	1,164	1,065
Beginning balance at original discount rate	1,222	1,155
Effect of changes in cash flow assumptions		0	0
Effect of actual variances from expected experience		(2)	(10)
Adjusted beginning of year balance		1,220	1,145
Issuances	60	177
Interest accrual	14	50
Benefit payments	(39)	(150)
Derecognition (lapses)	0	0
Ending balance at original discount rate	1,255	1,222
Effect of changes in discount rate assumptions	(75)	(58)
Ending balance	1,180	1,164
Adjustment due to reserve flooring	0	0
Reserve for future policy benefits	1,180	1,164
Less: reinsurance recoverable	845	880
Net liability for future policy benefits, after reinsurance recoverable	335	284
Discounted expected future gross premiums	0	0
Expected future gross premiums	0	0
Expected future benefit payments	$ 1,786	$ 1,726
Weighted average interest accretion rate	4.40%	4.20%
Weighted average discount rate	5.20%	4.90%
Weighted average duration of liability (in years)	7 years	7 years
Term and Whole Life Insurance
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	$ 703	$ 686
Beginning balance at original discount rate	708	708
Effect of changes in cash flow assumptions		0	(19)
Effect of actual variances from expected experience		(2)	(2)
Adjusted beginning of year balance		706	687
Issuances	16	55
Interest accrual	9	36
Net premiums collected	(18)	(70)
Derecognition (lapses)	0	0
Ending balance at original discount rate	713	708
Effect of changes in discount rate assumptions	(18)	(5)
Ending balance	695	703
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	1,325	1,319
Beginning balance at original discount rate	1,291	1,313
Effect of changes in cash flow assumptions		0	(18)
Effect of actual variances from expected experience		(2)	(1)
Adjusted beginning of year balance		1,289	1,294
Issuances	16	56
Interest accrual	18	73
Benefit payments	(34)	(132)
Derecognition (lapses)	0	0
Ending balance at original discount rate	1,289	1,291
Effect of changes in discount rate assumptions	6	34
Ending balance	1,295	1,325
Adjustment due to reserve flooring	6	5
Reserve for future policy benefits	606	627
Less: reinsurance recoverable	428	440
Net liability for future policy benefits, after reinsurance recoverable	178	187
Discounted expected future gross premiums	1,713	1,764
Expected future gross premiums	2,909	2,938
Expected future benefit payments	$ 2,163	$ 2,166
Weighted average interest accretion rate	6.30%	6.20%
Weighted average discount rate	5.30%	5.10%
Weighted average duration of liability (in years)	7 years	7 years
Disability Income Insurance
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	$ 104	$ 134
Beginning balance at original discount rate	105	137
Effect of changes in cash flow assumptions		0	(19)
Effect of actual variances from expected experience		(4)	(18)
Adjusted beginning of year balance		101	100
Issuances	2	12
Interest accrual	1	5
Net premiums collected	(2)	(12)
Derecognition (lapses)	0	0
Ending balance at original discount rate	102	105
Effect of changes in discount rate assumptions	(3)	(1)
Ending balance	99	104
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	661	696
Beginning balance at original discount rate	621	669
Effect of changes in cash flow assumptions		0	(25)
Effect of actual variances from expected experience		(6)	(29)
Adjusted beginning of year balance		615	615
Issuances	2	11
Interest accrual	9	37
Benefit payments	(10)	(42)
Derecognition (lapses)	0	0
Ending balance at original discount rate	616	621
Effect of changes in discount rate assumptions	26	40
Ending balance	642	661
Adjustment due to reserve flooring	0	0
Reserve for future policy benefits	543	557
Less: reinsurance recoverable	21	22
Net liability for future policy benefits, after reinsurance recoverable	522	535
Discounted expected future gross premiums	879	904
Expected future gross premiums	1,250	1,269
Expected future benefit payments	$ 1,054	$ 1,068
Weighted average interest accretion rate	6.30%	6.10%
Weighted average discount rate	5.40%	5.10%
Weighted average duration of liability (in years)	8 years	8 years
Long Term Care Insurance
Liability for Future Policy Benefit, Expected Net Premium [Roll Forward]
Beginning balance	$ 1,146	$ 1,207
Beginning balance at original discount rate	1,137	1,220
Effect of changes in cash flow assumptions		0	19
Effect of actual variances from expected experience		(3)	(3)
Adjusted beginning of year balance		1,134	1,236
Issuances	0	0
Interest accrual	14	59
Net premiums collected	(37)	(158)
Derecognition (lapses)	0	0
Ending balance at original discount rate	1,111	1,137
Effect of changes in discount rate assumptions	(6)	9
Ending balance	1,105	1,146
Liability for Future Policy Benefit, Expected Future Policy Benefit [Roll Forward]
Beginning balance	6,561	6,439
Beginning balance at original discount rate	6,507	6,569
Effect of changes in cash flow assumptions		0	9
Effect of actual variances from expected experience		(6)	5
Adjusted beginning of year balance		6,501	$ 6,583
Issuances	0	0
Interest accrual	81	329
Benefit payments	(104)	(405)
Derecognition (lapses)	0	0
Ending balance at original discount rate	6,478	6,507
Effect of changes in discount rate assumptions	(83)	54
Ending balance	6,395	6,561
Adjustment due to reserve flooring	0	0
Reserve for future policy benefits	5,290	5,415
Less: reinsurance recoverable	2,675	2,738
Net liability for future policy benefits, after reinsurance recoverable	2,615	2,677
Discounted expected future gross premiums	1,279	1,325
Expected future gross premiums	1,740	1,786
Expected future benefit payments	$ 10,741	$ 10,850
Weighted average interest accretion rate	5.10%	5.00%
Weighted average discount rate	5.40%	5.10%
Weighted average duration of liability (in years)	8 years	8 years