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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 | $ 17,500,000 us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet | $ 19,000,000 us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet | $ 6,200,000 us-gaap_AdjustmentToAdditionalPaidInCapitalIncomeTaxEffectFromShareBasedCompensationNet |
Net operating loss carryforwards for federal and state tax | 29,000,000 us-gaap_OperatingLossCarryforwards | 16,900,000 us-gaap_OperatingLossCarryforwards | 14,900,000 us-gaap_OperatingLossCarryforwards |
Company's total liability for uncertain tax positions | 17,165,000 us-gaap_UnrecognizedTaxBenefits | 14,902,000 us-gaap_UnrecognizedTaxBenefits | 13,072,000 us-gaap_UnrecognizedTaxBenefits | 16,165,000 us-gaap_UnrecognizedTaxBenefits
Uncertain tax positions recognized would favorably impact its effective tax rate | 16,100,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |  |  |
Increase in uncertain tax position | 2,300,000 md_IncreaseInUncertainTaxPosition | 1,800,000 md_IncreaseInUncertainTaxPosition |  |
Increase (decrease) of interest and penalties | 300,000 us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear | (600,000)us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear | 300,000 us-gaap_IncomeTaxExaminationIncreaseDecreaseInLiabilityFromPriorYear |
Company's accrued liability for interest and penalties related to income tax liabilities | 8,700,000 us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued | 8,400,000 us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued |  |
Accounts payable and accrued expenses | 25,900,000 us-gaap_LiabilityForUncertainTaxPositionsCurrent | 23,300,000 us-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 | 4,900,000 md_IncreaseInUncertainTaxPosition / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |
Decrease in uncertain tax position | $ 1,700,000 us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |

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