Retirement Plans - Allocation of Plan Assets, Pension Plan (Detail) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	$ 985.6	$ 878.8	$ 979.3
Total pension plan assets	985.6	878.8
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	258.4	236.6
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	14.6	16.1
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	0.1	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	239.4	219.4
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	4.3	1.0
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	727.2	642.2
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	366.2	324.7
Portion at Other than Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	316.2	274.0
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	32.1	31.2
Portion at Other than Fair Value Measurement | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	12.7	12.3
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	10.0	12.1
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	10.0	11.9
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	248.4	224.5
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	4.6	4.2
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	239.4	219.4
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	$ 4.3	$ 0.8