Employee Benefit Plans - Pre-Tax Amounts in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 228.5	$ 249.0
Total pre-tax amounts included in AOCL	228.5	249.0
Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	3.9	2.8
Total pre-tax amounts included in AOCL	$ 3.9	$ 2.8