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Equity Incentive Plans - Assumptions used in Black-Scholes Option-Pricing Model on Weighted Average Basis (Detail) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Grant-date fair value of option $ 4.30mx_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsGrantDateFairValuePerShare $ 2.70mx_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsGrantDateFairValuePerShare $ 4.12mx_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsGrantDateFairValuePerShare
Expected term 2 years 9 months 18 days 3 years 3 years
Risk-free interest rate 0.40%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 0.40%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 0.90%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
Expected volatility 37.60%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate 48.80%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate 52.20%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
Expected dividends 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate