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Revenue Recognition - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Deferred Revenue Associated With Remaining Performance Obligation	$ 6.9
Recognition Of Deferred Revenue	3.7
Additional Recognition Of Deferred Revenue	3.2
KHK License Agreement [Member]
Revenue Recognised With Performance Obligation	0.8
Helsinn License Agreement [Member]
Revenue Recognised With Performance Obligation	$ 0.4	$ 0.5

X

- Definition

The amount of additionally reported deferred or unearned revenue that was recognized as revenue during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	meip_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

No definition available.

+ Details

Name:	meip_RevenuePerformanceObligationAmount
Namespace Prefix:	meip_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of revenue recognized from performance obligation satisfied or partially satisfied in previous reporting periods. Includes, but is not limited to, change in transaction price.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 12A
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL108322424-203045

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130556-203045

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: