Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 10,514.5	$ 9,558.2
Operating Segments [Member] | U.S. Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	5,009.0	4,707.8
Operating Segments [Member] | International Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	5,002.4	3,984.7
Operating Segments [Member] | Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	356.9	444.8
Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 146.2	$ 420.9