Stock Based Compensation - Additional Information (Detail) - USD ($)	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	$ 14,591,000	$ 19,511,000	$ 15,527,000
Warrants outstanding	0	344,000
Stock Grants \| IPO
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 6,600,000	$ 8,500,000	4,000,000
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted, contractual term	10 years
Stock-based compensation expense	$ 8,031,000	10,977,000	$ 8,932,000
Unrecognized compensation expense related to non-vested stock options expected to be recognized	$ 14,600,000	$ 11,000,000
Unrecognized compensation expense related to non-vested stock options, weighted-average period of recognition	1 year 10 months 24 days	1 year 7 months 6 days
Total intrinsic value of stock options exercised	$ 4,200,000	$ 100,000
Stock Options \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted, vesting period	2 years
Stock Options \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted, vesting period	4 years

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Number of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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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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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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- Definition

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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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