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Stock-Based Compensation - Stock Option Activity (Detail) - Employee Stock Option - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Number of Shares
Stock Options, Beginning Balance	5,693,250
Stock Options, Options granted/exercised/forfeited	0
Stock Options, Ending Balance	5,693,250	5,693,250
Stock Options, Vested and Exercisable	5,230,375
Weighted Average Exercise Price
Weighted Average Exercise Price, Outstanding Beginning balance	$ 7.71
Weighted Average Exercise Price, Options granted/exercised/forfeited	0
Weighted Average Exercise Price, Outstanding Ending balance	7.71	$ 7.71
Weighted Average Exercise Price, Vested and Exercisable	$ 8.20
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Outstanding, Beginning balance	$ 8,987	$ 11,920
Aggregate Intrinsic Value, Vested and Exercisable	$ 8,192
Weighted Average Remaining Contractual Life
Weighted Average Remaining Contractual Life, Outstanding	5 years	5 years 3 months 18 days
Weighted Average Remaining Contractual Life, Vested and Exercisable	5 years 4 months 24 days

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Share based compensation arrangement by share-based payment award, options, aggregate intrinsic value.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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