Other Comprehensive (Loss) Income - Schedule of Components of Other Comprehensive (Loss) Income and Income Tax Expense (Benefit) Allocated to Each Component (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive loss, Before Tax Amount	$ (39.4)	$ (64.1)	$ (16.9)	$ (89.4)
Other comprehensive loss, Income Tax	0.5	0.3	1.5	(2.4)
Other comprehensive loss	(39.9)	(64.4)	(18.4)	(87.0)
Translation adjustments
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive loss, Before Tax Amount	(41.6)	20.1	(22.7)	29.1
Other comprehensive loss	(41.6)	20.1	(22.7)	29.1
Adjustment for net periodic pension and other postretirement benefits plan cost
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive loss, Before Tax Amount	2.2	1.0	5.8	2.0
Other comprehensive loss, Income Tax	0.5	0.3	1.5	0.6
Other comprehensive loss	$ 1.7	0.7	$ 4.3	1.4
Adjustments for available-for-sale securities
Accumulated Other Comprehensive Income Loss [Line Items]
Other comprehensive loss, Before Tax Amount		(85.2)		(120.5)
Other comprehensive loss, Income Tax				(3.0)
Other comprehensive loss		$ (85.2)		$ (117.5)