Benefit Plans - Plan Assets Using Fair Value Methodologies (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 31.0	$ 180.7
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		12.0
Fixed Income Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		137.5
Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9.3	158.8
Total | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.4
Total | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8.9	9.3
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.4	0.0
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Fair Value, Inputs, Level 1 | Fixed Income Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Fair Value, Inputs, Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.4
Fair Value, Inputs, Level 1 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	149.5
Fair Value, Inputs, Level 2 | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		12.0
Fair Value, Inputs, Level 2 | Fixed Income Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		137.5
Fair Value, Inputs, Level 2 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0
Fair Value, Inputs, Level 2 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8.9	9.3
Fair Value, Inputs, Level 3 | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Fair Value, Inputs, Level 3 | Fixed Income Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0.0
Fair Value, Inputs, Level 3 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0
Fair Value, Inputs, Level 3 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8.9	9.3
Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 21.7	$ 21.9