Investments - Pledged Investments and Reverse Repurchase Agreements (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Cash and investments at fair value on deposit with, or in trust accounts for the benefit of, various counterparties	$ 10,500.0	$ 7,900.0
Cash and investments at fair value on deposit with, or in trust accounts for the benefit of, U.S. state regulatory authorities	2,900.0	1,200.0
Reverse repurchase agreements	$ 159.7	$ 38.5