Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 10,734.3	$ 10,514.5	[1]
Operating Segments | U.S. Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	6,065.9	5,009.0
Operating Segments | International Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	4,140.8	5,002.4
Operating Segments | Run-off Lines
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	334.9	356.9
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 192.7	$ 146.2

[1] Derived from audited consolidated financial statements.