CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Other	$ 3,054	$ 1,303
Total cash and cash equivalents	53,773	37,836	$ 43,291	$ 30,574
Short-term investments	50,699	103,800
Ruble-denominated cash and cash equivalents
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Bank accounts	29,090	30,672
Short-term investments	15,309
U.S. dollar-denominated cash and cash equivalents
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Bank accounts	21,629	5,861
Short-term investments	$ 35,390	$ 103,800
Minimum
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Deposits in banks with original maturity period for classification as short-term investments	91 days
Minimum | Ruble-denominated cash and cash equivalents
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Annual interest rate (as a percent)	11.48%
Minimum | U.S. dollar-denominated cash and cash equivalents
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Annual interest rate (as a percent)	1.54%	0.55%
Maximum
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Deposits in banks with original maturity period for classification as short-term investments	365 days
Maximum | Ruble-denominated cash and cash equivalents
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Annual interest rate (as a percent)	15.00%
Maximum | U.S. dollar-denominated cash and cash equivalents
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Annual interest rate (as a percent)	4.44%	3.90%