Future Policy Benefits and Claims - Gross Premiums or Assessments and Interest Accretion (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Future Policy Benefits and Claims
Gross premiums or assessments	$ 1,015.3	$ 967.9	$ 1,707.1	$ 2,291.6
Interest accretion	637.2	550.5	1,186.9	1,102.3
Retirement and Income Solutions
Future Policy Benefits and Claims
Gross premiums or assessments	491.5	449.9	665.4	1,264.2
Interest accretion	371.6	357.7	745.2	708.5
Retirement and Income Solutions \| Pension risk transfer
Future Policy Benefits and Claims
Gross premiums or assessments	485.0	444.0	653.4	1,250.1
Interest accretion	324.6	308.1	650.6	608.6
Retirement and Income Solutions \| Individual fixed income annuities
Future Policy Benefits and Claims
Gross premiums or assessments	6.5	5.9	12.0	14.1
Interest accretion	47.0	49.6	94.6	99.9
Principal Asset Management \| International Pension \| Latin America \| Individual fixed income annuities
Future Policy Benefits and Claims
Gross premiums or assessments		4.1		5.9
Interest accretion	141.9	78.4	196.3	167.2
Benefits and Protection
Future Policy Benefits and Claims
Gross premiums or assessments	522.7	512.7	1,038.9	1,018.6
Interest accretion	121.4	112.1	240.7	221.9
Benefits and Protection \| Specialty Benefits \| Individual disability
Future Policy Benefits and Claims
Gross premiums or assessments	165.4	161.8	325.9	320.4
Interest accretion	27.1	26.2	54.0	52.0
Benefits and Protection \| Life Insurance \| Universal life
Future Policy Benefits and Claims
Gross premiums or assessments	179.2	177.2	360.6	354.2
Interest accretion	75.9	69.8	150.3	138.3
Benefits and Protection \| Life Insurance \| Term life
Future Policy Benefits and Claims
Gross premiums or assessments	178.1	173.7	352.4	344.0
Interest accretion	18.4	16.1	36.4	31.6
Corporate \| Long-term care insurance
Future Policy Benefits and Claims
Gross premiums or assessments	1.1	1.2	2.8	2.9
Interest accretion	$ 2.3	$ 2.3	$ 4.7	$ 4.7