Components of Net Periodic Benefit Expense Related to Post-Retirement Plans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plans Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4.3	$ 4.9	$ 9.2	$ 10.1
Interest cost	4.5	4.5	9.2	9.1
Expected return on plan assets	(4.2)	(4.2)	(8.3)	(8.5)
Amortization of net actuarial loss from earlier periods	2.1	2.3	4.5	4.9
Net periodic expense	6.7	7.5	14.6	15.6
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.6	0.6	1.2	1.1
Interest cost	0.3	0.2	0.6	0.5
Amortization of net prior service costs from earlier periods		(0.1)	(0.1)	(0.1)
Net periodic expense	$ 0.9	$ 0.7	$ 1.7	$ 1.5