Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ (202.9)
Amounts reclassified from accumulated other comprehensive (loss) income	1.2	$ 29.3
Other comprehensive income (loss)	21.5	(22.6)
Ending Balance	(188.3)
Changes in the Fair Value of Available-for-Sale Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance		119.3
Other comprehensive (loss) income before reclassifications		(60.9)
Amounts reclassified from accumulated other comprehensive (loss) income		28.6
Other comprehensive income (loss)		(32.3)
Ending Balance		87.0
Pension and Other Postretirement Benefits Plan Cost
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(144.6)	(159.5)
Other comprehensive (loss) income before reclassifications	1.4
Amounts reclassified from accumulated other comprehensive (loss) income	1.2	0.7
Other comprehensive income (loss)	2.6	0.7
Ending Balance	(142.0)	(158.8)
Translation Adjustments
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	40.9	(15.5)
Other comprehensive (loss) income before reclassifications	18.4	8.8
Other comprehensive income (loss)	18.4	8.8
Ending Balance	59.3	(6.7)
AOCI Attributable to Parent
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(103.7)	(55.7)
Other comprehensive (loss) income before reclassifications	19.8	(52.1)
Amounts reclassified from accumulated other comprehensive (loss) income	1.2	29.3
Other comprehensive income (loss)	21.0	(22.8)
Ending Balance	$ (82.7)	$ (78.5)