SHAREHOLDERS' EQUITY (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2004	Dec. 31, 2017	Dec. 31, 2016
Option Exercises
Options exercised during period (in shares)		1,000,892	711,863
Net proceeds from options exercised during the period		$ 7.0	$ 6.7
ESPP
Shareholders' equity
Number of shares authorized for issuance		3,000,000
Number of shares available for future issuance		409,049
Shares sold		261,569	201,264
Weighted average purchase price of shares sold (in dollars per share)		$ 7.49	$ 16.19
Proceeds from shares sold		$ 2.0	$ 3.3
ESPP | Minimum
Shareholders' equity
Purchase price expressed as percentage of fair market value of common stock	85.00%