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           Stock Based and Deferred Compensation (Details 1) (USD $) 
        In Thousands, except Share data, unless otherwise specified  | 
        3 Months Ended | 12 Months Ended | ||||
|---|---|---|---|---|---|---|
| 
           Mar. 31, 2015 
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           Dec. 31, 2014 
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| Summary of stock based compensation agreements | ||||||
| Unrecognized stock-based compensation cost | $ 31,410us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized | $ 20,577us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized | ||||
| Weighted average years expense recognition period | 2 years 10 months 28 days | 2 years 1 month 10 days | ||||
| Total equity awards outstanding | 1,203,218mth_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesOutstanding | [1] | 1,255,714mth_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesOutstanding | [1] | ||
          
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- Definition 
          Share based compensation arrangement by share based payment award number of shares outstanding. No definition available. 
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- Details 
          
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- Definition 
          Unrecognized cost of unvested share-based compensation awards. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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