Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 10,166.7	$ 9,558.2	[1]
Operating Segments [Member] | U.S. Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	5,234.6	4,707.8
Operating Segments [Member] | International Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	4,380.0	3,984.7
Operating Segments [Member] | Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	367.3	444.8
Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 184.8	$ 420.9

[1] Derived from audited consolidated financial statements.