Accumulated Other Comprehensive Loss - Schedule of Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance			$ 754.3
Balance as adjusted			754.3
Ending Balance	$ 873.4		873.4
Currency Translation
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(226.4)	$ (245.8)	(211.9)	$ (269.6)
Other comprehensive income (loss)	4.5	29.9	(10.0)	53.7
Ending Balance	(221.9)	(215.9)	(221.9)	(215.9)
Marketable Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		1.2	4.8	1.8
Change in accounting principle	(4.8)		(4.8)
Balance as adjusted		1.2		1.8
Other comprehensive income (loss)		(0.6)		(1.2)
Ending Balance		0.6		0.6
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(167.9)	(180.9)	(172.8)	(184.8)
Other comprehensive income (loss)	2.4	2.4	7.3	6.3
Ending Balance	(165.5)	(178.5)	(165.5)	(178.5)
Accumulated Defined Benefit Plans Adjustment | OPEB Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	1.0	1.6	1.2	1.8
Other comprehensive income (loss)			(0.2)	(0.2)
Ending Balance	1.0	1.6	1.0	1.6
Interest Rate Swap
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		(1.8)		(2.0)
Other comprehensive income (loss)		(0.7)		(1.5)
Reclassification adjustment for amounts included in earnings		2.5		3.5
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(393.3)	(425.7)	(378.7)	(452.8)
Change in accounting principle	(4.8)		(4.8)		$ (4.8)
Balance as adjusted	(393.3)	(425.7)	(383.5)	(452.8)
Other comprehensive income (loss)	6.9	33.5	(2.9)	60.6
Ending Balance	$ (386.4)	$ (392.2)	$ (386.4)	$ (392.2)