Schedule of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense recognized	$ 30,460	$ 20,481	$ 13,589
Unrecognized compensation cost	24,866
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense recognized	684	1,646	2,679
Unrecognized compensation cost	$ 292
Weighted-average period over which unrecognized compensation cost will be recognized:	1 year 1 month 6 days
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense recognized	$ 28,657	17,747	9,809
Unrecognized compensation cost	$ 24,116
Weighted-average period over which unrecognized compensation cost will be recognized:	2 years
Incentive Compensation Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense recognized	$ 661	442	376
Unrecognized compensation cost	$ 458
Weighted-average period over which unrecognized compensation cost will be recognized:	1 year 7 months 6 days
Directors' Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense recognized	$ 458	$ 646	$ 725

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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No definition available.

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