Components of Net Periodic Benefit Expense Related to Post-Retirement Plans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4.6	$ 5.0	$ 13.8	$ 15.1
Interest cost	4.7	4.5	13.9	13.6
Expected return on plan assets	(4.1)	(4.3)	(12.4)	(12.8)
Amortization of net actuarial loss from earlier periods	2.1	2.5	6.6	7.4
Amortization of net prior service costs from earlier periods		0.1		0.1
Net periodic expense	7.3	7.8	21.9	23.4
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.7	0.6	1.9	1.7
Interest cost	0.2	0.3	0.8	0.8
Amortization of net actuarial loss from earlier periods	0.1	0.1	0.1	0.1
Amortization of net prior service costs from earlier periods	(0.1)	(0.1)	(0.2)	(0.2)
Net periodic expense	$ 0.9	$ 0.9	$ 2.6	$ 2.4