Stock Incentive Plans and Stock Purchase Plan - Schedule of Activity and Certain Other Information Related to Stock Option Awards (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Shares, Outstanding at January 1, 2014	1,966,170
Number of Shares, Exercised	(1,151,576)
Number of Shares, Forfeited	(500)
Number of Shares, Outstanding at December 31, 2014	814,094	1,966,170
Number of Shares, Exercisable at December 31, 2014	814,094
Weighted Average Exercise Price, Outstanding at January 1, 2014	$ 26.74
Weighted Average Exercise Price, Exercised	$ 26.16
Weighted Average Exercise Price, forfeited	$ 27.72
Weighted Average Exercise Price, Outstanding at December 31, 2014	$ 27.57	$ 26.74
Weighted Average Exercise Price, Exercisable at December 31, 2014	$ 27.57
Weighted Average Remaining Contractual Term (in years), Outstanding at December 31, 2014	3 years 7 months 6 days
Weighted Average Remaining Contractual Term (in years), Exercisable at December 31, 2014	3 years 7 months 6 days
Aggregate Intrinsic Value, Exercised	$ 38.8	$ 35.4	$ 18.1
Aggregate Intrinsic Value, forfeited	0
Aggregate Intrinsic Value, Outstanding at December 31, 2014	31.4
Aggregate Intrinsic Value, Exercisable at December 31, 2014	$ 31.4

Stock Incentive Plans and Stock Purchase Plan - Schedule of Activity and Certain Other Information Related to Stock Option Awards (Detail) (USD $)
In Millions, except Share data, unless otherwise specified

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Number of Shares, Outstanding at January 1, 2014

1,966,170us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber

Number of Shares, Exercised

(1,151,576)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised

Number of Shares, Forfeited

(500)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod

Number of Shares, Outstanding at December 31, 2014

814,094us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber

1,966,170us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber

Number of Shares, Exercisable at December 31, 2014

814,094us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber

Weighted Average Exercise Price, Outstanding at January 1, 2014

$ 26.74us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice

Weighted Average Exercise Price, Exercised

$ 26.16us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice

Weighted Average Exercise Price, forfeited

$ 27.72us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice

Weighted Average Exercise Price, Outstanding at December 31, 2014

$ 27.57us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice

$ 26.74us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice

Weighted Average Exercise Price, Exercisable at December 31, 2014

$ 27.57us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice

Weighted Average Remaining Contractual Term (in years), Outstanding at December 31, 2014

3 years 7 months 6 days

Weighted Average Remaining Contractual Term (in years), Exercisable at December 31, 2014

3 years 7 months 6 days

Aggregate Intrinsic Value, Exercised

$ 38.8us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue

$ 35.4us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue

$ 18.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue

Aggregate Intrinsic Value, forfeited

0md_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueForfeited

Aggregate Intrinsic Value, Outstanding at December 31, 2014

31.4us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue

Aggregate Intrinsic Value, Exercisable at December 31, 2014

$ 31.4us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value Forfeited

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

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-Subparagraph (c)(1)(iii)

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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-Name Accounting Standards Codification

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- Definition

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)(iv)(3)

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Number of share options (or share units) exercised during the current period.

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