Retirement Plans and Other Post-Retirement Benefits - Schedule of Fair Value of Benefit Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 333.2	$ 372.8
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	41.8	65.2
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	291.4	307.6
Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	330.9	161.1
Fixed Income [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	41.8	13.3
Fixed Income [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	289.1	147.8
Cash and Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.3	6.7
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	$ 2.3	6.6
Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		119.0
Large Cap [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		6.9
Large Cap [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		112.1
Small and Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		25.6
Small and Mid Cap [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		25.6
International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		45.1
International Equity [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		4.0
International Equity [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		41.1
Real Estate Investment Trusts (REIT) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		15.3
Real Estate Investment Trusts (REIT) [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 15.3