Stock-Based Compensation - Effect of Recording Stock-Based Compensation (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
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

Allocated share based compensation effect on earning per share basic.

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Allocated share based compensation effect on earning per share diluted.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount, after tax, of expense for award under share-based payment arrangement.

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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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