CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Other	$ 651	$ 1,303
Total cash and cash equivalents	84,039	37,836	$ 29,837	$ 30,574
Short-term investments	17,200	103,800
Ruble-denominated cash and cash equivalents
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Bank accounts	16,266	30,672
U.S. dollar-denominated cash and cash equivalents
CASH AND CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS; OTHER NON-CURRENT ASSETS
Bank accounts	67,122	5,861
Short-term investments	$ 17,200	$ 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 | 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)	2.10%	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 | 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)	2.49%	3.90%