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Stock Incentive Plans and Stock Purchase Plan - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Shares available for future grants and awards under Stock Incentive Plans | 6,300,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant |  |
Stock-based compensation expense related to its stock incentive plans and stock purchase plan | $ 31,700,000 us-gaap_AllocatedShareBasedCompensationExpense | $ 31,300,000 us-gaap_AllocatedShareBasedCompensationExpense | $ 28,400,000 us-gaap_AllocatedShareBasedCompensationExpense
Aggregate fair value of restricted shares, vested | 28,200,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue | 33,300,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue | 23,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
Weighted average grant-date fair value of restricted and deferred stock awards granted | $ 57.73 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 46.48 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 30.06 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Stock-based compensation cost related to non-vested stock options remaining to be recognized as compensation expense, weighted-average period in years | 1 year 4 months 24 days |  |
Unrecognized Compensation Expense related to Nonvested Shares | 26,100,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions |  |
Aggregate intrinsic value of stock options exercised | 38,800,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | 35,400,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | 18,100,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Excess tax benefit related to employee stock incentive plans | 17,479,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation | 18,981,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation | 6,198,000 us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
Cash proceeds received from the exercise of stock options | $ 30,100,000 us-gaap_ProceedsAndExcessTaxBenefitFromSharebasedCompensation | $ 18,900,000 us-gaap_ProceedsAndExcessTaxBenefitFromSharebasedCompensation | $ 20,800,000 us-gaap_ProceedsAndExcessTaxBenefitFromSharebasedCompensation
Stock Option [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of years over which options become exercisable on a pro rata basis | 3 years |  |
Stock Option [Member] | Maximum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period of options, maximum years | 10 years |  |
Restricted Stock [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of years over which options become exercisable on a pro rata basis | 3 years |  |
1996 Non-Qualified Employee Stock Purchase Plan [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Percentage of market value of common stock at which employees are permitted to purchase | 85.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent / us-gaap_AwardTypeAxis = md_NineteenNinetySixNonQualifiedEmployeeStockPurchasePlanMember |  |
Percentage of discount received by participating employees | 15.00%md_ShareBasedCompensationArrangementByShareBasedDiscountReceivedByParticipatingEmployees / us-gaap_AwardTypeAxis = md_NineteenNinetySixNonQualifiedEmployeeStockPurchasePlanMember |  |
Shares reserved for issuance | 335,000 us-gaap_CommonStockCapitalSharesReservedForFutureIssuance / us-gaap_AwardTypeAxis = md_NineteenNinetySixNonQualifiedEmployeeStockPurchasePlanMember |  |
Shares issued under Stock Purchase Plan | 260,000 us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans / us-gaap_AwardTypeAxis = md_NineteenNinetySixNonQualifiedEmployeeStockPurchasePlanMember |  |

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- Definition Amount of increase in additional paid in capital (APIC) resulting from a tax benefit associated with share-based compensation plan other than an employee stock ownership plan (ESOP). Includes, but is not limited to, excess tax benefit. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 740 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6419406&loc=d3e23524-113945 + Details
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