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Stock Incentive Plans and Stock Purchase Plans - Additional Information (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]
Common stock, aggregate shares authorized	200,000,000	200,000,000
Stock-based compensation expense	$ 21.1	$ 33.4	$ 36.4
Aggregate fair value of restricted and deferred stocks vested	$ 59.3	$ 31.5	$ 27.4
Weighted average grant-date fair value of restricted and deferred stock awards granted	$ 23.54	$ 33.28	$ 51.99
Aggregate intrinsic value of stock options exercised		$ 0.6	$ 2.8
Cash proceeds received from the exercise of stock options		$ 0.1	$ 3.7
Options Grants In Period Weighted Average Grant Date Fair Value	$ 5.21
Expected volatility rate	50.00%
Expected term	3 years
Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Unrecognized compensation expense related to non-vested restricted stock, weighted-average period in years	2 years 7 months 6 days
Stock Option, Share Based Compensation Cost Not Yet Recognized Period For Recognition	$ 4.3
Stock Option [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options, maximum years	10 years
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Restricted and Deferred Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to non-vested restricted stock, weighted-average period in years	1 year 9 months 18 days
Unrecognized compensation expense related to non-vested restricted shares	$ 11.6
1996 Non-Qualified Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of market value of common stock at which employees are permitted to purchase	85.00%
2015 Non-Qualified Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of market value of common stock at which employees are permitted to purchase	90.00%
Amended and Restated 2008 Plan [Member] | Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants and awards under Stock Incentive Plans	5,000,000.0
1996 Non-Qualified Employee Stock Purchase Plan and 2015 Non-Qualified Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, aggregate shares authorized	2,600,000
Aggregate number Shares issued under Stock Purchase Plans	500,000
Common stock, reserved for issuance	500,000

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

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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