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Fair Value Measurement of Plan Assets (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | $ 873.8 | $ 785.6 |
Non-investment plan assets | 1.0 | 1.4 |
Total plan assets | 874.8 | 787.0 |
Short-term Investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 15.8 | 28.7 |
Equity Securities | Commingled Equity Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 121.5 | 105.6 |
Equity Securities | Common Stock |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 220.3 | 254.0 |
Fixed Income Securities | Other Fixed Income Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 219.9 | 102.6 |
Fixed Income Securities | Commingled Fixed Income Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 99.8 | 90.3 |
Fixed Income Securities | US Treasury Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 46.0 | 97.6 |
Other Investments | Private equity |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 20.9 | 28.0 |
Other Investments | Mutual Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 51.8 | 4.5 |
Other Investments | Real estate funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 77.8 | 74.3 |
Quoted Prices In Active Markets For Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 268.3 | 352.7 |
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 | 2.0 | 1.1 |
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 | 220.3 | 254.0 |
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 | 46.0 | 97.6 |
Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 506.8 | 310.9 |
Significant Other Observable Inputs (Level 2) | Short-term Investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 13.8 | 7.9 |
Significant Other Observable Inputs (Level 2) | Equity Securities | Commingled Equity Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 121.5 | 105.6 |
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 | 219.9 | 102.6 |
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 | 99.8 | 90.3 |
Significant Other Observable Inputs (Level 2) | Other Investments | Mutual Fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 51.8 | 4.5 |
Significant Non-observable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 98.7 | 122.0 | $ 117.6
Significant Non-observable Inputs (Level 3) | Short-term Investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments |  | 19.7 | 12.4
Significant Non-observable Inputs (Level 3) | Real estate funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 77.8 | 74.3 | $ 70.9
Significant Non-observable Inputs (Level 3) | Other Investments | Private equity |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | 20.9 | 28.0 |
Significant Non-observable Inputs (Level 3) | Other Investments | Real estate funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total fair value of plan asset investments | $ 77.8 | $ 74.3 |

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