Components of Net Pension (Expense) Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Retirement Benefits [Abstract]
Expected employer contribution	$ 5.8		$ 5.8
Components of net pension expense
Service cost	1.3	$ 1.1	2.5	$ 2.3
Interest cost	2.8	3.0	5.6	5.9
Expected return on plan assets	(3.3)	(3.2)	(6.7)	(6.5)
Recognized net actuarial loss	1.1	1.2	2.3	2.4
Net pension expense	$ 1.9	$ 2.1	$ 3.7	$ 4.1