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Income Taxes - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Tax Contingency [Line Items]        
Effective tax rate 37.63%us-gaap_EffectiveIncomeTaxRateContinuingOperations 37.44%us-gaap_EffectiveIncomeTaxRateContinuingOperations 37.94%us-gaap_EffectiveIncomeTaxRateContinuingOperations  
Excess tax benefits related to stock-based awards, which increased the cash flow from financing activities $ 17,500,000us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet $ 19,000,000us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet $ 6,200,000us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet  
Net operating loss carryforwards for federal and state tax 29,000,000us-gaap_OperatingLossCarryforwards 16,900,000us-gaap_OperatingLossCarryforwards 14,900,000us-gaap_OperatingLossCarryforwards  
Company's total liability for uncertain tax positions 17,165,000us-gaap_UnrecognizedTaxBenefits 14,902,000us-gaap_UnrecognizedTaxBenefits 13,072,000us-gaap_UnrecognizedTaxBenefits 16,165,000us-gaap_UnrecognizedTaxBenefits
Uncertain tax positions recognized would favorably impact its effective tax rate 16,100,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate      
Increase in uncertain tax position 2,300,000md_IncreaseInUncertainTaxPosition 1,800,000md_IncreaseInUncertainTaxPosition    
Increase (decrease) of interest and penalties 300,000us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear (600,000)us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear 300,000us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear  
Company's accrued liability for interest and penalties related to income tax liabilities 8,700,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued 8,400,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued    
Accounts payable and accrued expenses 25,900,000us-gaap_LiabilityForUncertainTaxPositionsCurrent 23,300,000us-gaap_LiabilityForUncertainTaxPositionsCurrent    
Earliest Tax Year [Member]        
Income Tax Contingency [Line Items]        
Federal and state income tax examinations 2004      
Latest Tax Year [Member]        
Income Tax Contingency [Line Items]        
Federal and state income tax examinations 2013      
Minimum [Member]        
Income Tax Contingency [Line Items]        
Expiration period of operating loss carryforwards 2019      
Maximum [Member]        
Income Tax Contingency [Line Items]        
Expiration period of operating loss carryforwards 2034      
Scenario, Forecast [Member]        
Income Tax Contingency [Line Items]        
Increase in uncertain tax position 4,900,000md_IncreaseInUncertainTaxPosition
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		      
Decrease in uncertain tax position $ 1,700,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember