Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 9,876.1	$ 10,317.8
Operating Segments | U.S. Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	5,585.2	5,800.1
Operating Segments | International Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,859.3	3,932.3
Operating Segments | Run-off Lines
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	287.9	314.7
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 143.7	$ 270.7