Future Policy Benefits and Claims - Gross Premiums or Assessments and Interest Accretion (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Future Policy Benefits and Claims
Gross premiums or assessments	$ 967.9	$ 1,524.3	$ 2,291.6	$ 2,793.1
Interest accretion	550.5	526.1	1,102.3	1,023.5
Retirement and Income Solutions
Future Policy Benefits and Claims
Gross premiums or assessments	449.9	1,002.3	1,264.2	1,772.3
Interest accretion	357.7	332.3	708.5	658.4
Retirement and Income Solutions \| Pension risk transfer
Future Policy Benefits and Claims
Gross premiums or assessments	444.0	991.1	1,250.1	1,744.1
Interest accretion	308.1	279.7	608.6	552.6
Retirement and Income Solutions \| Individual fixed income annuities
Future Policy Benefits and Claims
Gross premiums or assessments	5.9	11.2	14.1	28.2
Interest accretion	49.6	52.6	99.9	105.8
Principal Asset Management \| International Pension \| Latin America \| Individual fixed income annuities
Future Policy Benefits and Claims
Gross premiums or assessments	4.1	10.9	5.9	13.0
Interest accretion	78.4	91.7	167.2	162.8
Benefits and Protection
Future Policy Benefits and Claims
Gross premiums or assessments	512.7	509.9	1,018.6	1,005.1
Interest accretion	112.1	99.8	221.9	197.8
Benefits and Protection \| Specialty Benefits \| Individual disability
Future Policy Benefits and Claims
Gross premiums or assessments	161.8	159.6	320.4	314.5
Interest accretion	26.2	24.6	52.0	49.0
Benefits and Protection \| Life Insurance \| Universal life
Future Policy Benefits and Claims
Gross premiums or assessments	177.2	180.7	354.2	356.9
Interest accretion	69.8	61.6	138.3	122.0
Benefits and Protection \| Life Insurance \| Term life
Future Policy Benefits and Claims
Gross premiums or assessments	173.7	169.6	344.0	333.7
Interest accretion	16.1	13.6	31.6	26.8
Corporate \| Long-term care insurance
Future Policy Benefits and Claims
Gross premiums or assessments	1.2	1.2	2.9	2.7
Interest accretion	$ 2.3	$ 2.3	$ 4.7	$ 4.5