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NET INCOME (LOSS) PER SHARE - Narrative (Details) (Stock options and restricted stock units)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Stock options and restricted stock units
      
Schedule of Earnings Per Share [Line Items]      
Total number of options to purchase common stock and restricted stock units, outstanding (in shares) 2.3mtsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOutstandingNumber
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= mtsn_StockOptionsAndRestrictedStockUnitsMember		 4.7mtsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOutstandingNumber
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= mtsn_StockOptionsAndRestrictedStockUnitsMember		 6.5mtsn_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOutstandingNumber
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= mtsn_StockOptionsAndRestrictedStockUnitsMember