Stock Compensation - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unissued common stock available for grant under the plans (in shares)	2,300,000
Share-based compensation expense	$ 10.0	$ 8.6
Grants of stock option awards in period (in shares)	0	0
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 9.9	$ 7.8
Aggregate weighted-average grant-date fair value of units granted	17.9	14.3
Total unrecognized compensation cost	$ 15.6
Unrecognized compensation cost, weighted-average recognition period	2 years 1 month 10 days
Aggregate weighted-average grant-date fair value of units vested	$ 10.3	7.3
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 0.1	$ 0.8
Performance-Based Units \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares which will vest after achievement of goals (in shares)	0
Performance-Based Units \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares which will vest after achievement of goals (in shares)	2

Share Based Compensation Arrangement By Share Based Payment Award Number of Shares Vesting After Achievement of Goals

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- Definition

Share-based compensation arrangement by share-based payment award, other than options, grants in period, weighted average grant date fair value.

+ References

No definition available.

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- Definition

Share-based compensation arrangement by share-based payment award, other than options, vested in period, weighted average grant date fair value.

+ References

No definition available.

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- Definition

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.

+ References

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

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- Definition

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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- Definition

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.

+ References

No definition available.

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- Definition

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.

+ References

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-Publisher FASB
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- Definition

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

+ References

No definition available.

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