Investments - Offsetting of Assets and Liabilities (Details) - Free-standing derivatives - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Asset derivatives:
Gross Amounts	$ 23.1	$ 19.0
Gross Amounts Offset in the Consolidated Balance Sheets	0.0	0.0
Net Amounts of Assets/ Liabilities Presented in the Consolidated Balance Sheets	23.1	19.0
Gross Amounts Not Offset in the Consolidated Balance Sheets, Financial Instruments	0.0	0.0
Gross Amounts Not Offset in the Consolidated Balance Sheets, Cash Collateral Received	23.6	18.5
Net Amount	$ (0.5)
Net Amount		$ 0.5