Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (1,326.5)
Other comprehensive income before reclassifications	(6.0)
Amounts reclassified from accumulated other comprehensive income	38.7	$ 34.9
Current period other comprehensive income	32.7
Ending balance	(1,270.8)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3,815.8)
Ending balance	(3,783.1)
Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(817.0)
Other comprehensive income before reclassifications	(5.8)
Amounts reclassified from accumulated other comprehensive income	0.0
Current period other comprehensive income	(5.8)
Ending balance	(822.8)
Postretirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(2,998.8)
Other comprehensive income before reclassifications	(0.2)
Amounts reclassified from accumulated other comprehensive income	38.7
Current period other comprehensive income	38.5
Ending balance	$ (2,960.3)