Retirement Plans - Allocation of Plan Assets, Postretirement Benefit Plan (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value and net asset value of the company's benefit plan Investments	$ 210.3	$ 205.5
Estimate of Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	22.7	6.6
Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	21.3	5.4
Estimate of Fair Value Measurement | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	1.4	1.2
Portion at Other than Fair Value Measurement
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	187.6	198.9
Portion at Other than Fair Value Measurement | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	149.8	143.5
Portion at Other than Fair Value Measurement | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Benefit plan investments measured at net asset value	37.8	55.4
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	1.2
Fair Value, Inputs, Level 1 | Estimate of Fair Value Measurement | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of the company's benefit plan assets	0.0	0.0
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	1.2
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	21.3	5.4
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	21.3	5.4
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	$ 0.0