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STOCK-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Stock-based compensation |  |  |
Stock-based compensation expense | $ 12,500 | $ 6,100 |
Valuation assumptions |  |  |
Risk-free interest rates (as a percent) | 2.79% |  |
Expected volatility (as a percent) | 61.80% |  |
Additional disclosures |  |  |
Market price (in dollars per share) | $ 40.33 |  |
Total intrinsic value of options exercised | $ 19,100 | 200 |
Weighted Average Grant Date Fair Value |  |  |
Unrecognized compensation cost | $ 137,600 |  |
Weighted average period for recognition of unrecognized compensation cost | 3 years 2 months 12 days |  |
Share based compensation expense | $ 12,500 | $ 6,100 |
Stock Options |  |  |
Valuation assumptions |  |  |
Risk-free interest rates (as a percent) |  | 2.13% |
Expected term | 6 years 5 months 9 days | 6 years 7 months 2 days |
Expected volatility (as a percent) |  | 62.90% |
Dividend yield (as a percent) | 0.00% | 0.00% |
Weighted average fair value per share of options granted during the period (in dollars per share) | $ 26.84 | $ 13.20 |
Shares |  |  |
Outstanding at the beginning of the period (in shares) | 3,360,461 |  |
Granted (in shares) | 343,566 |  |
Exercised (in shares) | (420,129) |  |
Outstanding at the end of the period (in shares) | 3,283,898 |  | 3,360,461
Exercisable at the end of the period (in shares) | 1,757,384 |  |
Weighted Average Exercise Price |  |  |
Outstanding at the beginning of the period (in dollars per share) | $ 11.89 |  |
Granted (in dollars per share) | 44.37 |  |
Exercised (in dollars per share) | 3.31 |  |
Outstanding at the end of the period (in dollars per share) | 16.39 |  | $ 11.89
Exercisable at the end of the period (in dollars per share) | $ 10.80 |  |
Weighted Average Remaining Contractual Term |  |  |
Outstanding at the end of the period | 7 years |  | 6 years 4 months 24 days
Exercisable at the end of the period | 5 years 4 months 24 days |  |
Aggregate Intrinsic Value |  |  |
Outstanding at the end of the period | $ 80,098 |  |
Exercisable at the end of the period | $ 51,901 |  |
Restricted Stock Awards And Restricted Stock Units RSU |  |  |
Valuation assumptions |  |  |
Weighted average fair value per share of stock purchase rights granted during the period (in dollars per share) | $ 45.16 |  |
Restricted Shares |  |  |
Outstanding at the beginning of the period (in shares) | 6,148,778 |  |
Granted (in shares) | 1,079,223 |  |
Released (in shares) | (880,022) |  |
Forfeited (in shares) | (33,307) |  |
Outstanding at the end of the period (in shares) | 6,314,672 |  | 6,148,778
Weighted Average Grant Date Fair Value |  |  |
Outstanding at the beginning of the period (in dollars per share) | $ 15.76 |  |
Granted (in dollars per share) | 45.16 |  |
Released (in dollars per share) | 17.67 |  |
Forfeited (in dollars per share) | 26.37 |  |
Outstanding at the end of the period (in dollars per share) | $ 20.36 |  | $ 15.76
ASU 2016-09 |  |  |
Stock-based compensation |  |  |
Cumulative-effect adjustment |  | $ 400 |

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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + 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 (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + 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)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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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 + Details
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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 + Details
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- Definition Net number of share options (or share units) granted during 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 (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + 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 (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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