Investment Securities - Lloyd's Trust Deposits, Bank Loan Fund, and Insurance-Linked Securities Funds (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 1,643.0	$ 1,488.0
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	179.5	174.8
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	148.7	137.4
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 123.6	$ 49.3
Minimum | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Redemptions and advanced redemption notice period	45 days
Minimum | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Redemptions and advanced redemption notice period	30 days
Maximum | Hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Redemptions and advanced redemption notice period	90 days
Maximum | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Redemptions and advanced redemption notice period	90 days