Benefit Plans - Plan Assets Using Fair Value Methodologies (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 33.7	$ 31.0
Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10.1	9.3
Total | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.1	0.4
Total | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10.0	8.9
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.1	0.4
Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.1	0.4
Level 1 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Level 2 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Level 2 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10.0	8.9
Level 3 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Level 3 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10.0	8.9
Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 23.6	$ 21.7