Postretirement Benefit Plans - Amount Recognized in Accumulated Other Comprehensive Income Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 235.5	$ 232.1
Prior service cost (benefit)	(51.8)	(67.6)
Total	183.7	164.5
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	168.9	169.1
Prior service cost (benefit)	5.6	6.6
Total	174.5	175.7
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	66.6	63.0
Prior service cost (benefit)	(57.4)	(74.2)
Total	$ 9.2	$ (11.2)