Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 8,764.0	$ 7,205.0
U.S. Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	4,298.4	3,961.2
International Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,553.8	2,356.9
Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	449.6	537.0
Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 462.2	$ 349.9