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Postretirement Benefit Plans - Fair Value of Plan Assets Held by Our Postretirement Benefits Plans (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset | $ 0.5 | $ 321.8 | $ 281.3
Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset | 0.5 | 320.5 |
Pension Plan [Member] | Fixed income - collective trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 238.8 |
Pension Plan [Member] | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 0.0 |
Pension Plan [Member] | Cash and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 81.7 |
Pension Plan [Member] | Level 1 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 81.7 |
Pension Plan [Member] | Level 1 [Member] | Fixed income - collective trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 0.0 |
Pension Plan [Member] | Level 1 [Member] | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 0.0 |
Pension Plan [Member] | Level 1 [Member] | Cash and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 81.7 |
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 238.8 |
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | Fixed income - collective trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 238.8 |
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 0.0 |
Pension Plan [Member] | Fair Value Measured at Net Asset Value Per Share [Member] | Cash and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 0.0 |
Other Postretirement Benefits Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset | $ 0.0 | 1.3 | $ 2.9
Other Postretirement Benefits Plan [Member] | Fixed income - collective trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 0.0 |
Other Postretirement Benefits Plan [Member] | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | 1.3 |
Other Postretirement Benefits Plan [Member] | Cash and other |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan asset |  | $ 0.0 |

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