Retirement Plans - Components of Net Pension and Postretirement Benefits (Income) Expense and Total (Income) Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Settlements and curtailments	$ 1.6	$ 79.3
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.7	1.0	$ 1.7
Interest cost	31.6	105.7	170.4
Expected return on plan assets	(50.3)	(177.5)	(234.6)
Amortization of prior service credit	0.0	0.0	0.0
Amortization of actuarial (gain) loss	7.3	26.1	$ 40.5
Settlements and curtailments	$ 1.6	$ 98.4
Discount rate	3.80%	4.30%	3.90%
Expected return on plan assets	5.90%	6.80%	7.00%
Pension Benefits | Discontinued Operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income) expense		$ (43.3)	$ 16.8
Pension Benefits | Continuing operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income) expense	$ (9.1)	10.4	(5.2)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.3	3.8	4.7
Interest cost	11.1	11.7	15.9
Expected return on plan assets	(13.5)	(13.8)	(13.1)
Amortization of prior service credit	(2.8)	(12.7)	(26.9)
Amortization of actuarial (gain) loss	$ (0.1)	0.1
Settlements and curtailments		$ (19.5)	$ 0.0
Discount rate	4.00%	4.20%	3.90%
Expected return on plan assets	6.80%	7.30%	7.30%
Other Postretirement Benefits | Continuing operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income) expense	$ (4.0)	$ (30.4)	$ (19.4)