Benefit Plans - Pension Plans with Accumulated Benefit Obligation or Projected Benefit Obligation in Excess of Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension plans with a projected benefit obligation in excess of plan assets:
Projected benefit obligation	$ 111.7	$ 277.9
Fair value of plan assets	30.4	180.7
Pension plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation	111.7	270.6
Fair value of plan assets	30.4	173.7
Accumulated benefit obligation	$ 107.1	$ 268.3