Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Contingent purchase price	$ 14,344	$ 14,655
Rempex \| Event-based milestones
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Contingent purchase price	$ 19,198	$ 19,650
Rempex \| Event-based milestones \| Probability-adjusted discounted cash flow
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Probability of success	47.00%	71.00%
Rempex \| Event-based milestones \| Minimum \| Probability-adjusted discounted cash flow
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Probability of success	18.00%	18.00%
Discount rate	4.80%	4.80%
Rempex \| Event-based milestones \| Maximum \| Probability-adjusted discounted cash flow
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Probability of success	100.00%	90.00%
Discount rate	7.30%	7.50%

Noncurrent portion of the fair value, as of the balance sheet date, of potential payments under the contingent consideration arrangements.

No definition available.

+ Details

Name:	mdco_ContingentPurchasePrice
Namespace Prefix:	mdco_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueInputsAssetsQuantitativeInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

+ References

+ Details

Name:	us-gaap_FairValueInputsProbabilityOfDefault
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=mdco_RempexPharmaceuticalsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mdco_MilestoneAxis=mdco_EventBasedMilestoneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=mdco_AdjustedDiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: