Employee Share Plans - Schedule of Share-Based Compensation Awards (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Compensation And Retirement Disclosure [Abstract]
Stock options	$ 531	$ 527	$ 2,078	$ 1,578
Restricted stock awards	978	930	3,280	2,669
Total stock-based compensation expense	$ 1,509	$ 1,457	$ 5,358	$ 4,247

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section S99
-Paragraph 1
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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- Definition

The noncash expense that accounts for the value of stock or unit options distributed to employees as compensation.

+ References

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