Share-Based Payments - Assumptions Used in Determining Fair Value of RSU Awards Granted (Detail) - Restricted Stock Units - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology
Grant date fair value	$ 4.57	$ 7.02	$ 7.53
Grant date closing stock price	5.10	6.74	$ 11.05
Grant date closing index value	$ 6,687.92	$ 4,941.53
Volatility	47.00%	48.66%	48.90%
Risk-free rate	1.49%	0.85%	0.84%
Probability of death or disability			5.00%

Share based compensation arrangement by share based payment award fair value assumptions and methodology.

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- Definition

Share based compensation arrangement by share based payment award fair value assumptions grant date closing index value.

+ References

No definition available.

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- Definition

Share based compensation Arrangement by share based payment award fair value assumptions grant date closing stock price

+ References

No definition available.

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- Definition

Share based compensation arrangement by share based payment award fair value assumptions probability of death or disability.

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- Definition

Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

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No definition available.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

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

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

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

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