Employee Benefits - Components of Estimated Net Pension and Other Postretirement Benefit Plan (Income) Expense (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension (income) expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.6	$ 0.5
Interest cost	44.8	47.7
Expected return on plan assets	(61.6)	(63.1)
Amortization, net	10.2	7.8
Net periodic benefit (income) expense	(6.0)	(7.1)
Other postretirement benefits plan (income) expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.2	1.1
Interest cost	4.0	4.2
Expected return on plan assets	(3.3)	(3.1)
Amortization, net	(6.7)	(6.4)
Net periodic benefit (income) expense	$ (4.8)	$ (4.2)