Retirement Plans - Allocation of Plan Assets, Postretirement Benefit Plan (Detail) - OPEB - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value and net asset value of the company's benefit plan Investments	$ 237.2	$ 227.7
Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	69.7	50.3
Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	37.0	19.7
Estimate of Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	31.3	30.2
Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	1.4	0.4
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	167.5	177.4
Portion at Other than Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	2.6	15.6
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	164.9	161.8
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	1.4	0.4
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	1.4	0.4
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	68.3	49.9
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	37.0	19.7
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	$ 31.3	$ 30.2