Retirement Plans - Allocation of Plan Assets, Postretirement Benefit Plan (Detail) - OPEB - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Fair value and net asset value of the company's benefit plan Investments	$ 247.6	$ 237.2
Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	85.4	69.7
Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	54.2	37.0
Estimate of Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	31.2	31.3
Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets		1.4
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	162.2	167.5
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	2.6
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	159.6	164.9
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
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
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	85.4	68.3
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	54.2	37.0
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	$ 31.2	$ 31.3