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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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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Unrecognized cost of unvested options awarded to employees as compensation.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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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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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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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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Gross number of share options (or share units) granted during the period.

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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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Number of options outstanding, including both vested and non-vested options.

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Number of options vested.

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Share-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019
Stock Option Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option awards granted	23,000
Options outstanding, shares	1,459,000		1,459,000		1,460,000
2013 Equity Plan \| Stock Option Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vested	40,625
Aggregate fair value of stock option vested	$ 2.4
Weighted average grant date fair value of stock option awards	$ 48.84	$ 50.76
Share-based compensation expense recognized	$ 3.0	$ 3.4	$ 6.0	$ 8.1
Unrecognized share-based compensation expense	$ 28.4		$ 28.4
Unrecognized compensation expense expected to be recognized			2 years 8 months 12 days
2013 Equity Plan \| Other Stock-based Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense expected to be recognized			2 years 10 months 24 days
Other stock-based awards granted	0
Unrecognized share-based compensation expense	$ 0.4		$ 0.4
2013 Equity Plan \| Two Executive Officers \| Stock Option Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized share-based compensation expense	$ 4.2		$ 4.2
Unrecognized compensation expense expected to be recognized			2 years 9 months 18 days
2013 Equity Plan \| Class A \| Stock Option Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option awards granted	22,500
Options outstanding, shares	1,458,733		1,458,733
2013 Equity Plan \| Class A \| Employees, Officers, and Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, stock authorized	2,300,000		2,300,000
Shares of class A common stock reserved and available for future issuance	572,500		572,500