Future Policy Benefits and Claims - Interest Accretion (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
Interest accretion	$ 533.0	$ 538.0	$ 1,719.9	$ 1,640.3
Retirement and Income Solutions
Future Policy Benefits and Claims
Interest accretion	373.9	359.8	1,119.1	1,068.3
Retirement and Income Solutions \| Pension risk transfer
Future Policy Benefits and Claims
Interest accretion	327.6	310.8	978.2	919.4
Retirement and Income Solutions \| Individual fixed income annuities
Future Policy Benefits and Claims
Interest accretion	46.3	49.0	140.9	148.9
Principal Asset Management \| International Pension \| Latin America \| Individual fixed income annuities
Future Policy Benefits and Claims
Interest accretion	32.4	60.9	228.7	228.1
Benefits and Protection
Future Policy Benefits and Claims
Interest accretion	124.5	114.9	365.2	336.8
Benefits and Protection \| Specialty Benefits \| Individual disability
Future Policy Benefits and Claims
Interest accretion	27.8	26.4	81.8	78.4
Benefits and Protection \| Life Insurance \| Universal life
Future Policy Benefits and Claims
Interest accretion	77.6	71.2	227.9	209.5
Benefits and Protection \| Life Insurance \| Term life
Future Policy Benefits and Claims
Interest accretion	19.1	17.3	55.5	48.9
Corporate \| Long-term care insurance
Future Policy Benefits and Claims
Interest accretion	$ 2.2	$ 2.4	$ 6.9	$ 7.1