Retirement Plans - Allocation of Plan Assets, Postretirement Benefit Plan (Detail) - OPEB - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value and net asset value of the company's benefit plan Investments	$ 227.7	$ 208.9
Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	50.3	36.8
Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	19.7	36.6
Estimate of Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	30.2
Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	0.4	0.2
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	177.4	172.1
Portion at Other than Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	15.6	47.3
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	161.8	124.8
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	0.4	0.2
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	0.4	0.2
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	49.9	36.6
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	19.7	$ 36.6
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	$ 30.2