Segment Information - Schedule of Identifiable Assets (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 8,816.6	$ 8,503.1
Operating Segments | U.S. Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	6,788.6	6,279.0
Operating Segments | International Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	1,705.9	1,888.9
Operating Segments | Run-off Lines
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	215.7	221.4
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 106.4	$ 113.8