Benefit Plans - Plan Assets Using Fair Value Methodologies (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 425.7	$ 390.1
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.8	3.4
Fixed income | Foreign Government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	289.5	290.5
Fixed income | US Treasury Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	43.2	30.5
Fixed income | Collateralized Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.3	4.5
Fixed income | Other securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63.1	37.0
Equity securities | U S Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1.2	2.2
Equity securities | International Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10.2	8.3
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12.4	13.7
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed income | Foreign Government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed income | US Treasury Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed income | Collateralized Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed income | Other securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 1 | Equity securities | U S Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 1 | Equity securities | International Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 1 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	424.6	388.1
Fair Value, Inputs, Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.8	3.4
Fair Value, Inputs, Level 2 | Fixed income | Foreign Government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	289.5	290.5
Fair Value, Inputs, Level 2 | Fixed income | US Treasury Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	43.2	30.5
Fair Value, Inputs, Level 2 | Fixed income | Collateralized Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.3	4.5
Fair Value, Inputs, Level 2 | Fixed income | Other securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63.1	37.0
Fair Value, Inputs, Level 2 | Equity securities | U S Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1.2	2.2
Fair Value, Inputs, Level 2 | Equity securities | International Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10.2	8.3
Fair Value, Inputs, Level 2 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.3	11.7
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1.1	2.0
Fair Value, Inputs, Level 3 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed income | Foreign Government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed income | US Treasury Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed income | Collateralized Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed income | Other securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Equity securities | U S Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Equity securities | International Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Inputs, Level 3 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1.1	$ 2.0