Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension [Member]
Pension And Other Employee Benefit Plans [Line Items]
Service cost	$ 1.3	$ 1.7	$ 3.9	$ 4.7
Interest cost	2.8	2.4	8.4	7.2
Expected return on plan assets	(3.2)	(3.2)	(9.5)	(9.2)
Amortization of:
Prior service benefits	0	0	0	0
Net actuarial loss	1.4	2.0	4.9	6.0
Net periodic cost (benefit)	2.3	2.9	7.7	8.7
Postretirement [Member]
Pension And Other Employee Benefit Plans [Line Items]
Service cost	0	0.1	0	0.2
Interest cost	0.3	0.3	0.8	0.9
Expected return on plan assets	0	0.4	0	0.4
Amortization of:
Prior service benefits	(1.3)	(1.3)	(4.1)	(4.1)
Net actuarial loss	0.2	0.2	0.5	0.8
Net periodic cost (benefit)	$ (0.8)	$ (0.3)	$ (2.8)	$ (1.8)