Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 8,240.7	$ 7,205.0	[1]
U.S. Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	4,127.7	3,961.2
International Operations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,223.6	2,356.9
Run Off Lines [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	414.6	537.0
Corporate and Other [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 474.8	$ 349.9

[1] Derived from audited consolidated financial statements.