Retirement Plans and Other Post-Retirement Benefits - Schedule of Fair Value of Benefit Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 673.1	$ 610.7
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	117.5	153.2
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	555.6	457.5
Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	214.9	201.9
Large Cap [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12.4	7.1
Large Cap [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	202.5	194.8
Small and Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46.2	50.9
Small and Mid Cap [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46.2	50.9
International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81.4	79.5
International Equity [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.2	38.8
International Equity [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	74.2	40.7
Real Estate Investment Trusts (REIT) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27.6	27.9
Real Estate Investment Trusts (REIT) [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27.6	27.9
Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	290.9	237.7
Fixed Income [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24.0	28.5
Fixed Income [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	266.9	209.2
Cash and Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12.1	12.8
Cash and Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.1
Cash and Equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 12.0	$ 12.8