Investments - Variable Interest Entity (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Variable Interest Entity
Total Assets	$ 26,072.3	$ 26,375.1
Total Liabilities	12,714.1	12,940.0
RMMC/RMBC
Variable Interest Entity
Total Assets	243.1	261.6
Total Liabilities	22.5	24.7
Other
Variable Interest Entity
Total Assets	3.2	2.8
Total Liabilities	$ 3.3	$ 3.3