Investment Securities - Lloyd's Trust Deposits, Bank Loan Fund, and Insurance-Linked Securities Funds (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 3,594.4	$ 2,796.2	$ 2,533.4	$ 2,746.0
Minimum
Debt and Equity Securities, FV-NI [Line Items]
Redemptions and advanced redemption notice period	30 days
Maximum
Debt and Equity Securities, FV-NI [Line Items]
Redemptions and advanced redemption notice period	90 days
Lloyds Bank
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 111.2
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	161.7
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 53.9