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Supplemental Disclosures of Cash Flow Information - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items] |  |  |
AMT credit and other U.S. federal refunds | $ (28.5) | $ (63.6) | $ (2.4)
Qualified Contributory Savings and Thrift 401(k) Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Matching contributions by employer, percentage | 100.00% |  |
Percentage of eligible compensation for matching contributions by employer | 5.00% |  |
Matching contributions vesting schedule | 5 years |  |
Contribution expense to plan | $ 63.6 | $ 59.4 | $ 53.9

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