Contractholder Funds - Guaranteed Minimum Interest Rate (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Excess of crediting rates over GMIR
Total policyholder account value	$ 25,376.3	$ 25,322.2
Retirement and Income Solutions
Excess of crediting rates over GMIR
Total policyholder account value	18,821.1	18,774.0
Workplace savings and retirement solutions | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	11,060.0	10,733.0
Policyholder account value without GMIR	1,920.6	1,988.5
Total policyholder account value	12,980.6	12,721.5	$ 12,154.7
Workplace savings and retirement solutions | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,451.4	1,423.9
Workplace savings and retirement solutions | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	6,875.5	7,166.5
Workplace savings and retirement solutions | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	2,708.9	2,118.3
Workplace savings and retirement solutions | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	7.4	7.4
Workplace savings and retirement solutions | 4.01% and above | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	16.8	16.9
Workplace savings and retirement solutions | At GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	5,110.1	5,516.4
Workplace savings and retirement solutions | At GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	4,751.4	5,135.0
Workplace savings and retirement solutions | At GMIR | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	334.5	357.1
Workplace savings and retirement solutions | At GMIR | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	7.4	7.4
Workplace savings and retirement solutions | At GMIR | 4.01% and above | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	16.8	16.9
Workplace savings and retirement solutions | Up to 0.50% above GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	0.1	7.2
Workplace savings and retirement solutions | Up to 0.50% above GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR		3.3
Workplace savings and retirement solutions | Up to 0.50% above GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR		3.8
Workplace savings and retirement solutions | Up to 0.50% above GMIR | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	0.1	0.1
Workplace savings and retirement solutions | 0.51% to 1.00% above GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,124.1	1,255.7
Workplace savings and retirement solutions | 0.51% to 1.00% above GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR		101.8
Workplace savings and retirement solutions | 0.51% to 1.00% above GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,122.4	1,153.1
Workplace savings and retirement solutions | 0.51% to 1.00% above GMIR | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1.7	0.8
Workplace savings and retirement solutions | 1.01% to 2.00% above GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,273.3	1,065.1
Workplace savings and retirement solutions | 1.01% to 2.00% above GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,108.8	1,006.0
Workplace savings and retirement solutions | 1.01% to 2.00% above GMIR | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	164.5	59.1
Workplace savings and retirement solutions | 2.01% or more above GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	3,552.4	2,888.6
Workplace savings and retirement solutions | 2.01% or more above GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	342.6	312.8
Workplace savings and retirement solutions | 2.01% or more above GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,001.7	874.6
Workplace savings and retirement solutions | 2.01% or more above GMIR | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	$ 2,208.1	$ 1,701.2
Workplace savings and retirement solutions | Minimum | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	1.01%	1.01%
Workplace savings and retirement solutions | Minimum | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	2.01%	2.01%
Workplace savings and retirement solutions | Minimum | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	3.01%	3.01%
Workplace savings and retirement solutions | Minimum | 4.01% and above | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	4.01%	4.01%
Workplace savings and retirement solutions | Maximum | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	1.00%	1.00%
Workplace savings and retirement solutions | Maximum | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	2.00%	2.00%
Workplace savings and retirement solutions | Maximum | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	3.00%	3.00%
Workplace savings and retirement solutions | Maximum | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	4.00%	4.00%
Individual variable annuities | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	$ 288.0	$ 300.7
Policyholder account value without GMIR	350.9	213.5
Total policyholder account value	638.9	514.2	381.4
Individual variable annuities | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	21.8	22.6
Individual variable annuities | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	4.8	4.6
Individual variable annuities | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	261.4	273.5
Individual variable annuities | At GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	288.0	300.7
Individual variable annuities | At GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	21.8	22.6
Individual variable annuities | At GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	4.8	4.6
Individual variable annuities | At GMIR | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	$ 261.4	$ 273.5
Individual variable annuities | Minimum | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	1.01%	1.01%
Individual variable annuities | Minimum | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	2.01%	2.01%
Individual variable annuities | Minimum | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	3.01%	3.01%
Individual variable annuities | Minimum | 4.01% and above | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	4.01%	4.01%
Individual variable annuities | Maximum | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	1.00%	1.00%
Individual variable annuities | Maximum | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	2.00%	2.00%
Individual variable annuities | Maximum | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	3.00%	3.00%
Individual variable annuities | Maximum | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	4.00%	4.00%
Individual fixed deferred annuities | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	$ 4,978.4	$ 5,300.1
Policyholder account value without GMIR	223.2	238.2
Total policyholder account value	5,201.6	5,538.3	7,228.3
Individual fixed deferred annuities | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,881.6	1,991.6
Individual fixed deferred annuities | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	197.1	253.9
Individual fixed deferred annuities | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	2,745.7	2,897.7
Individual fixed deferred annuities | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	154.0	156.9
Individual fixed deferred annuities | At GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	3,275.7	3,460.9
Individual fixed deferred annuities | At GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	282.8	305.5
Individual fixed deferred annuities | At GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	93.4	100.8
Individual fixed deferred annuities | At GMIR | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	2,745.5	2,897.7
Individual fixed deferred annuities | At GMIR | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	154.0	156.9
Individual fixed deferred annuities | Up to 0.50% above GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	98.6	116.3
Individual fixed deferred annuities | Up to 0.50% above GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	97.7	115.4
Individual fixed deferred annuities | Up to 0.50% above GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	0.9	0.9
Individual fixed deferred annuities | 0.51% to 1.00% above GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	127.7	148.1
Individual fixed deferred annuities | 0.51% to 1.00% above GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	103.3	121.8
Individual fixed deferred annuities | 0.51% to 1.00% above GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	24.2	26.3
Individual fixed deferred annuities | 0.51% to 1.00% above GMIR | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	0.2
Individual fixed deferred annuities | 1.01% to 2.00% above GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	461.7	572.9
Individual fixed deferred annuities | 1.01% to 2.00% above GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	386.6	449.4
Individual fixed deferred annuities | 1.01% to 2.00% above GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	75.1	123.5
Individual fixed deferred annuities | 2.01% or more above GMIR | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,014.7	1,001.9
Individual fixed deferred annuities | 2.01% or more above GMIR | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,011.2	999.5
Individual fixed deferred annuities | 2.01% or more above GMIR | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
Policyholder account value with GMIR	$ 3.5	$ 2.4
Individual fixed deferred annuities | Minimum | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	1.01%	1.01%
Individual fixed deferred annuities | Minimum | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	2.01%	2.01%
Individual fixed deferred annuities | Minimum | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	3.01%	3.01%
Individual fixed deferred annuities | Minimum | 4.01% and above | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	4.01%	4.01%
Individual fixed deferred annuities | Maximum | Up to 1.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	1.00%	1.00%
Individual fixed deferred annuities | Maximum | 1.01% - 2.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	2.00%	2.00%
Individual fixed deferred annuities | Maximum | 2.01% - 3.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	3.00%	3.00%
Individual fixed deferred annuities | Maximum | 3.01% - 4.00% | Retirement and Income Solutions
Excess of crediting rates over GMIR
GMIR	4.00%	4.00%
Universal life | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	$ 6,030.8	$ 6,067.7
Policyholder account value without GMIR	882.6	842.7
Total policyholder account value	6,913.4	6,910.4	$ 6,947.9
Universal life | Up to 1.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	19.7	19.5
Universal life | 1.01% - 2.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,627.3	1,613.2
Universal life | 2.01% - 3.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	2,550.3	2,596.8
Universal life | 3.01% - 4.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,779.8	1,784.0
Universal life | 4.01% and above | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	53.7	54.2
Universal life | At GMIR | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	2,648.9	2,721.3
Universal life | At GMIR | 1.01% - 2.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	289.2	294.1
Universal life | At GMIR | 2.01% - 3.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	707.8	729.7
Universal life | At GMIR | 3.01% - 4.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,615.8	1,657.0
Universal life | At GMIR | 4.01% and above | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	36.1	40.5
Universal life | Up to 0.50% above GMIR | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	730.2	730.9
Universal life | Up to 0.50% above GMIR | 2.01% - 3.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	667.1	677.2
Universal life | Up to 0.50% above GMIR | 3.01% - 4.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	59.6	49.8
Universal life | Up to 0.50% above GMIR | 4.01% and above | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	3.5	3.9
Universal life | 0.51% to 1.00% above GMIR | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	1,291.4	1,316.4
Universal life | 0.51% to 1.00% above GMIR | Up to 1.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	11.8	16.1
Universal life | 0.51% to 1.00% above GMIR | 1.01% - 2.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	421.0	418.1
Universal life | 0.51% to 1.00% above GMIR | 2.01% - 3.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	811.8	836.3
Universal life | 0.51% to 1.00% above GMIR | 3.01% - 4.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	38.6	37.6
Universal life | 0.51% to 1.00% above GMIR | 4.01% and above | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	8.2	8.3
Universal life | 1.01% to 2.00% above GMIR | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	930.3	875.1
Universal life | 1.01% to 2.00% above GMIR | Up to 1.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	5.0	1.0
Universal life | 1.01% to 2.00% above GMIR | 1.01% - 2.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	494.0	485.6
Universal life | 1.01% to 2.00% above GMIR | 2.01% - 3.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	362.7	350.6
Universal life | 1.01% to 2.00% above GMIR | 3.01% - 4.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	62.7	36.4
Universal life | 1.01% to 2.00% above GMIR | 4.01% and above | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	5.9	1.5
Universal life | 2.01% or more above GMIR | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	430.0	424.0
Universal life | 2.01% or more above GMIR | Up to 1.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	2.9	2.4
Universal life | 2.01% or more above GMIR | 1.01% - 2.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	423.1	415.4
Universal life | 2.01% or more above GMIR | 2.01% - 3.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	0.9	3.0
Universal life | 2.01% or more above GMIR | 3.01% - 4.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
Policyholder account value with GMIR	$ 3.1	$ 3.2
Universal life | Minimum | 1.01% - 2.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
GMIR	1.01%	1.01%
Universal life | Minimum | 2.01% - 3.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
GMIR	2.01%	2.01%
Universal life | Minimum | 3.01% - 4.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
GMIR	3.01%	3.01%
Universal life | Minimum | 4.01% and above | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
GMIR	4.01%	4.01%
Universal life | Maximum | Up to 1.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
GMIR	1.00%	1.00%
Universal life | Maximum | 1.01% - 2.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
GMIR	2.00%	2.00%
Universal life | Maximum | 2.01% - 3.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
GMIR	3.00%	3.00%
Universal life | Maximum | 3.01% - 4.00% | Benefits and Protection | Life Insurance
Excess of crediting rates over GMIR
GMIR	4.00%	4.00%