Retirement Plans - Allocation of Plan Assets, Pension Plan (Detail) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	$ 1,090.2	$ 985.6	$ 878.8
Total pension plan assets	1,090.2	985.6
Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	528.5	258.4
Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	18.9	14.6
Estimate of Fair Value Measurement | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	52.1	0.1
Estimate of Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	446.6	239.4
Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	10.9	4.3
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	561.7	727.2
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	372.6	366.2
Portion at Other than Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	145.5	316.2
Portion at Other than Fair Value Measurement | Hedge Funds and Other
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	30.4	32.1
Portion at Other than Fair Value Measurement | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	13.2	12.7
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	296.9	10.0
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	12.5	10.0
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	52.0
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	232.2
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.2
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	231.6	248.4
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	6.4	4.6
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.1	0.1
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	214.4	239.4
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	$ 10.7	$ 4.3