Goodwill and Other Intangible Assets - Changes in Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Changes in Goodwill
Beginning balance, Goodwill	$ 1,598.2	$ 1,627.6
Goodwill impairment		(27.1)
Foreign currency, Goodwill	10.3	(2.3)
Ending balance, Goodwill	1,608.5	1,598.2
Retirement and Income Solutions
Changes in Goodwill
Beginning balance, Goodwill	675.9	675.9
Ending balance, Goodwill	675.9	675.9
Principal Asset Management
Changes in Goodwill
Beginning balance, Goodwill	892.8	895.1
Foreign currency, Goodwill	10.3	(2.3)
Ending balance, Goodwill	903.1	892.8
Benefits and Protection
Changes in Goodwill
Beginning balance, Goodwill	29.5	56.6
Goodwill impairment		(27.1)
Ending balance, Goodwill	$ 29.5	$ 29.5