Future Policy Benefits and Claims - Gross Premiums or Assessments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Future Policy Benefits and Claims
Gross premiums or assessments	$ 1,419.0	$ 1,098.8	$ 3,126.1	$ 3,390.4
Retirement and Income Solutions
Future Policy Benefits and Claims
Gross premiums or assessments	892.5	569.1	1,557.9	1,833.3
Retirement and Income Solutions \| Pension risk transfer
Future Policy Benefits and Claims
Gross premiums or assessments	888.5	560.0	1,541.9	1,810.1
Retirement and Income Solutions \| Individual fixed income annuities
Future Policy Benefits and Claims
Gross premiums or assessments	4.0	9.1	16.0	23.2
Principal Asset Management \| International Pension \| Latin America \| Individual fixed income annuities
Future Policy Benefits and Claims
Gross premiums or assessments		0.5		6.4
Benefits and Protection
Future Policy Benefits and Claims
Gross premiums or assessments	525.3	528.0	1,564.2	1,546.6
Benefits and Protection \| Specialty Benefits \| Individual disability
Future Policy Benefits and Claims
Gross premiums or assessments	170.4	169.0	496.3	489.4
Benefits and Protection \| Life Insurance \| Universal life
Future Policy Benefits and Claims
Gross premiums or assessments	176.0	182.1	536.6	536.3
Benefits and Protection \| Life Insurance \| Term life
Future Policy Benefits and Claims
Gross premiums or assessments	178.9	176.9	531.3	520.9
Corporate \| Long-term care insurance
Future Policy Benefits and Claims
Gross premiums or assessments	$ 1.2	$ 1.2	$ 4.0	$ 4.1