Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 8,204.1	$ 10,034.4
Operating Segments | U.S. Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	6,024.1	5,815.0
Operating Segments | International Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	1,786.6	3,791.6
Operating Segments | Run-off Lines
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	266.4	284.4
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 127.0	$ 143.4