Benefit Plans - Pension Plans with Accumulated Benefit Obligation or Projected Benefit Obligation in Excess of Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension plans with a projected benefit obligation in excess of plan assets:
Projected benefit obligation	$ 120.8	$ 111.7
Fair value of plan assets	33.7	30.4
Pension plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation	120.8	111.7
Fair value of plan assets	33.7	30.4
Accumulated benefit obligation	$ 119.3	$ 107.1