Retirement Plans - Allocation of Plan Assets, Pension Plan (Detail) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	$ 878.8	$ 979.3	$ 875.4
Total pension plan assets	878.8	979.3
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	236.6	388.1
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	16.1	14.3
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	125.0
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	219.4	246.7
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	1.0	2.1
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	642.2	591.2
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	324.7	279.7
Portion at Other than Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	274.0	268.7
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	31.2	6.1
Portion at Other than Fair Value Measurement | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	12.3	36.7
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	12.1	137.1
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	11.9	11.5
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		124.9
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		0.7
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	224.5	251.0
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	4.2	2.8
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	219.4	246.0
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	$ 0.8	$ 2.1