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Investments - Schedule of Contractual Maturities of Long-Term Investments (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]    
Due after one year through five years $ 54,959us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount $ 41,655us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsNetCarryingAmount
Due after five years through seven years 9,123md_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughSevenYearsNetCarryingAmount 15,856md_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughSevenYearsNetCarryingAmount
Total $ 64,082us-gaap_HeldToMaturitySecuritiesNoncurrent $ 57,511us-gaap_HeldToMaturitySecuritiesNoncurrent