Benefit Plans - Plan Assets Using Fair Value Methodologies (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 390.1	$ 374.3
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.4	3.1
Fixed income | Foreign Government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	290.5	248.6
Fixed income | US Treasury Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30.5	52.0
Fixed income | Collateralized Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4.5	5.8
Fixed income | Other securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37.0	37.2
Equity securities | U S Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.2	2.4
Equity securities | International Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8.3	7.8
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13.7	17.4
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	388.1	370.2
Fair Value, Inputs, Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.4	3.1
Fair Value, Inputs, Level 2 | Fixed income | Foreign Government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	290.5	248.6
Fair Value, Inputs, Level 2 | Fixed income | US Treasury Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30.5	52.0
Fair Value, Inputs, Level 2 | Fixed income | Collateralized Mortgage Backed Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4.5	5.8
Fair Value, Inputs, Level 2 | Fixed income | Other securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37.0	37.2
Fair Value, Inputs, Level 2 | Equity securities | U S Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.2	2.4
Fair Value, Inputs, Level 2 | Equity securities | International Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8.3	7.8
Fair Value, Inputs, Level 2 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11.7	13.3
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.0	4.1
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	$ 2.0	$ 4.1