Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amortization of:
Prior service benefit	$ (59.5)	$ (64.9)
Pension
Components of net periodic cost:
Service cost	7.9	5.2	$ 6.1
Interest cost	11.3	11.1	9.6
Expected return on plan assets	(13.8)	(12.6)	(12.2)
Amortization of:
Prior service benefit	0.0	0.0	0.0
Net actuarial loss	10.9	6.3	8.1
Net periodic cost (benefit)	16.3	10.0	11.6
Postretirement
Components of net periodic cost:
Service cost	0.0	0.0	0.4
Interest cost	1.1	1.1	1.2
Expected return on plan assets	0.0	0.0	(0.2)
Amortization of:
Prior service benefit	(5.4)	(5.5)	(5.5)
Net actuarial loss	0.6	0.6	1.0
Net periodic cost (benefit)	$ (3.7)	$ (3.8)	$ (3.1)