Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 9,105.7	$ 8,764.0	[1]
Operating Segments [Member] | U.S. Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	4,490.6	4,298.4
Operating Segments [Member] | International Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,762.7	3,553.8
Operating Segments [Member] | Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	453.8	449.6
Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 398.6	$ 462.2

[1] Derived from audited consolidated financial statements.