Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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No definition available.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Percentage of vesting of share-based compensation awards.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Stock-Based Compensation - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 6.4	$ 2.6	$ 20.5	$ 14.5
Unamortized compensation expense expected to be vested	35.4		35.4
Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	0.4		1.2
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	2.3		5.0
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	2.3		7.0
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1.2		$ 6.4
Award vesting period (in years)			3 years
Performance Shares \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of incentive stock awards expected to be vested and awarded of stocks granted (as a percent)			0.00%
Performance Shares \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of incentive stock awards expected to be vested and awarded of stocks granted (as a percent)			200.00%
Performance Shares \| Scenario Forecast
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of incentive stock awards expected to be vested and awarded of stocks granted (as a percent)					91.00%	81.00%	71.00%
2011 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant (in shares)	3,800,000		3,800,000
2011 Plan \| Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period (in years)			4 years
Plan term (in years)			10 years
2011 Plan \| Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period (in years)			4 years
Directors Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 0.2		$ 0.9
Directors Plan \| Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period (in years)			1 year
2000 Plan \| Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period (in years)			4 years
Plan term (in years)			10 years
2000 Plan \| Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period (in years)			4 years