Summary of significant accounting policies - Changes in ROU assets (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Jan. 01, 2019
Changes in operating lease right-of-use asset
New operating lease right-of-use asset	$ 401
Amortization of right-of-use asset during the nine-month period ending September 30, 2019	(129)
Operating lease right-of use asset	593	$ 321
Operating lease expenses	$ 202

Amount of lessee's right to use underlying asset under operating lease for new operating lease.

No definition available.

Name:	mist_NewOperatingLeaseRightOfUseAsset
Namespace Prefix:	mist_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of amortization expense attributable to right-of-use asset from operating lease.

+ References

No definition available.

+ Details

Name:	mist_OperatingLeaseRightOfUseAssetAmortization
Namespace Prefix:	mist_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

n/a

+ References

No definition available.

+ Details

Name:	mist_RightOfUseAssetsRollforwardAbstract
Namespace Prefix:	mist_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of operating lease expense. Excludes sublease income.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918638-209977

+ Details

Name:	us-gaap_OperatingLeaseExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

Name:	us-gaap_OperatingLeaseRightOfUseAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant