Fair Value Measurement of Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	$ 821.0	$ 814.0
Non-investment plan assets	2.9	8.3
Total plan assets	823.9	822.3
Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	23.1	15.4
Other Investments | Private equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	34.8	37.7
Other Investments | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	64.1	57.2
Equity Securities | Commingled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	134.7	147.5
Equity Securities | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	264.4	272.9
Fixed Income Securities | Other Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	93.2	75.3
Fixed Income Securities | Commingled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	83.8	71.8
Fixed Income Securities | US Treasury Securities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	122.9	136.2
Quoted Prices In Active Markets For Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	389.7	410.5
Quoted Prices In Active Markets For Identical Assets (Level 1) | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	2.4	1.4
Quoted Prices In Active Markets For Identical Assets (Level 1) | Equity Securities | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	264.4	272.9
Quoted Prices In Active Markets For Identical Assets (Level 1) | Fixed Income Securities | US Treasury Securities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	122.9	136.2
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	311.7	294.3
Significant Other Observable Inputs (Level 2) | Equity Securities | Commingled Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	134.7	147.5
Significant Other Observable Inputs (Level 2) | Fixed Income Securities | Other Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	93.2	75.0
Significant Other Observable Inputs (Level 2) | Fixed Income Securities | Commingled Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	83.8	71.8
Significant Non-observable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	119.6	109.2	58.0
Significant Non-observable Inputs (Level 3) | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	64.1	57.2
Significant Non-observable Inputs (Level 3) | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	20.7	14.0
Significant Non-observable Inputs (Level 3) | Other Investments | Private equity
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	34.8	37.7
Significant Non-observable Inputs (Level 3) | Other Investments | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments	64.1	57.2
Significant Non-observable Inputs (Level 3) | Fixed Income Securities | Other Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan asset investments		$ 0.3