Retirement Plans - Allocation of Plan Assets, Postretirement Benefit Plan (Detail) - OPEB - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value and net asset value of the company's benefit plan Investments	$ 208.9	$ 228.6
Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	36.8	30.7
Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	36.6	30.7
Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	0.2	0.0
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	172.1	197.9
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	124.8	166.3
Portion at Other than Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	47.3	31.6
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	0.2
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	0.2
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	36.6	30.7
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	$ 36.6	30.7
Fair Value, Inputs, Level 2 | Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets		$ 0.0