Accumulated Other Comprehensive Income - Amounts Related to Postretirement Plans Not Reclassified in Entirely out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net of tax				$ 108.4
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of postretirement plan items	[1]	$ (1.4)	$ (0.8)	(4.2)	$ (2.3)
Amortization of actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of postretirement plan items	[1]	38.6	47.8	116.6	143.1
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of postretirement plan items		37.2	47.0	112.4	140.8
Income tax benefit		(1.1)	(3.9)	(4.0)	(6.9)
Net of tax		$ 36.1	$ 43.1	$ 108.4	$ 133.9

[1] These items are included in net periodic postretirement cost (see note (c)).