Retirement Plans - Allocation of Plan Assets, Pension Plan (Detail) - Pension Benefits - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	$ 979.3	$ 875.4	$ 3,424.1
Total pension plan assets	979.3	875.4
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	388.1	387.1
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	14.3	17.5
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	125.0	144.6
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	246.7	222.7
Estimate of Fair Value Measurement | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	2.1	2.3
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	591.2	488.3
Portion at Other than Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	268.7	193.4
Portion at Other than Fair Value Measurement | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	6.1	16.6
Portion at Other than Fair Value Measurement | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	36.7	32.8
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at net asset value	279.7	245.5
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	137.1	158.1
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	11.5	12.9
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	144.5
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	0.7
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	0.0	0.0
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	251.0	229.0
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	2.8	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	246.0	222.0
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Derivatives and Other
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value of plan assets, excluding net asset value of plan assets	$ 2.1	$ 2.3