SHARE BASED COMPENSATION (Tables)
 12 Months Ended
Dec. 31, 2017
SHARE BASED COMPENSATION [Abstract]  
Summary of Stock Option Activity
 
 
Number of
Share
Options
 
Weighted
Average
Exercise Price
US$
 
Weighted
Average
Remaining
Contractual
Years
 
Aggregate
Intrinsic
Value US$
 
Outstanding at December 31, 2014
 
 
8,038,950
 
 
1.44
 
 
 
 
 
 
 
Granted
 
 
615,624
 
 
3.38
 
 
 
 
 
 
 
Exercised
 
 
(3,015,872)
 
 
1.20
 
 
 
 
 
 
 
Forfeited
 
 
(56,926)
 
 
3.33
 
 
 
 
 
 
 
Expired
 
 
—
 
 
—
 
 
 
 
 
 
 
Outstanding at December 31, 2015
 
 
5,581,776
 
 
1.77
 
 
5.68
 
 
48,412
 
Granted
 
 
1,115,807
 
 
1.90
 
 
 
 
 
 
 
Exercised
 
 
(2,106,043)
 
 
1.46
 
 
 
 
 
 
 
Forfeited
 
 
(263,671)
 
 
2.98
 
 
 
 
 
 
 
Expired
 
 
—
 
 
—
 
 
 
 
 
 
 
Outstanding at December 31, 2016
 
 
4,327,869
 
 
1.88
 
 
5.84
 
 
56,743
 
Granted
 
 
682,435
 
 
0.88
 
 
 
 
 
 
 
Exercised
 
 
(1,898,391)
 
 
1.74
 
 
 
 
 
 
 
Forfeited
 
 
(100,279)
 
 
3.21
 
 
 
 
 
 
 
Expired
 
 
—
 
 
—
 
 
 
 
 
 
 
Outstanding at December 31, 2017
 
 
3,011,634
 
 
1.70
 
 
5.83
 
 
40,031
 
Vested and expected to vest as of December 31, 2017
 
 
2,716,474
 
 
1.57
 
 
5.61
 
 
36,454
 
Exercisable as of December 31, 2017
 
 
1,724,139
 
 
1.15
 
 
4.30
 
 
23,860
 
Summary of Fair Value Assumptions
 
The Company calculated the fair value of the share options on the grant date using the Binomial option-pricing valuation model. The assumptions used in the valuation model are summarized in the following table.
 
 
 
Year Ended December 31,
 
 
2015
 
2016
 
2017
 
 
US$
 
US$
 
US$
Expected volatility
 
 
62.3%-70.9%
 
 
63.5%-68.8%
 
 
63.7%-67.8%
Expected dividends yield
 
 
0%
 
 
0%
 
 
0%
Exercise multiple
 
 
2.0
 
 
2.0
 
 
2.0
Risk-free interest rate per annum
 
 
2.68%-2.99%
 
 
2.15%-3.18%
 
 
2.88%-3.22%
The fair value of underlying ordinary shares (per share)
 
 
US$9.28-US$12.85
 
 
US$9.99-US$16.54
 
 
US$14.27-US$19.03
Schedule of Fair Values of the Options Granted
 
 
Year Ended December 31,
 
 
 
2015
 
2016
 
2017
 
 
 
US$
 
US$
 
US$
 
Weighted average grant date fair value of option per share
 
 
9.16
 
 
10.17
 
 
15.99
 
Aggregate grant date fair value of options
 
 
5,641
 
 
11,352
 
 
10,915
 
Summary of Non-vested Shares Activity
 
 
Number of Non-
vested Shares
 
Weighted Average 
Grant Date Fair 
Value
 
 
 
 
 
 
US$
 
 
 
 
 
 
 
 
 
Outstanding as of December 31, 2014
 
 
27,780
 
 
9.06
 
Granted
 
 
25,000
 
 
9.63
 
Vested
 
 
(27,780)
 
 
9.06
 
Forfeited
 
 
-
 
 
-
 
Outstanding as of December 31, 2015
 
 
25,000
 
 
9.63
 
Granted
 
 
20,000
 
 
10.82
 
Vested
 
 
(35,000)
 
 
9.97
 
Forfeited
 
 
-
 
 
-
 
Outstanding as of December 31, 2016
 
 
10,000
 
 
10.82
 
Granted
 
 
129,835
 
 
14.93
 
Vested
 
 
(17,367)
 
 
14.08
 
Forfeited
 
 
(15,701)
 
 
14.47
 
Outstanding as of December 31, 2017
 
 
106,767
 
 
14.75