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Stock-Based Compensation - Effect of Recording Stock-Based Compensation (Detail) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Stock-based compensation expense | $ 29.5 | $ 49.2 | $ 27.3
Windfall tax effect on stock-based compensation | (2.2) | (2.2) | (8.3)
Net effect on net income | $ 27.3 | $ 47.0 | $ 19.0
Basic | $ 0.50 | $ 0.86 | $ 0.35
Diluted | $ 0.49 | $ 0.85 | $ 0.35
Restricted Stock Units (RSUs) [Member] |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Stock-based compensation expense | $ 27.0 | $ 47.0 | $ 24.9
Stock Appreciation Rights (SARs) [Member] |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Stock-based compensation expense |  | 0.1 | 0.1
Employee Stock Purchase Plan [Member] |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Stock-based compensation expense | $ 2.5 | $ 2.1 | $ 2.3

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- Definition Amount of expense for award under share-based payment arrangement. Excludes amount capitalized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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