Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Unrecognized cost of unvested options awarded to employees as compensation.

+ References

+ Details

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

- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

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

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

- Definition

The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(iii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=insm_EmployeeAndDirectorsStockOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range5Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range6Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range7Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range8Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range9Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=insm_Range10Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock-Based Compensation - Range of Exercise Prices (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock options to purchase common stock
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Total intrinsic value of stock options exercised during the period	$ 5.6	$ 4.3	$ 1.0
Unrecognized compensation expense related to unvested stock options	$ 29.7
Expected weighted average period for recognizing unrecognized compensation expense	2 years 4 months 24 days
$3.03 to $4.55
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	$ 3.03
Range of Exercise Prices upper range limit (in dollars per share)	$ 4.55
Number of Options outstanding (in shares)	960,038
Weighted Average Remaining Contractual Term (in years)	3 years 7 months 13 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 3.60
Number of Options exercisable (in shares)	960,038
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 3.60
$6.90 to $6.90
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	6.90
Range of Exercise Prices upper range limit (in dollars per share)	$ 6.90
Number of Options outstanding (in shares)	120,403
Weighted Average Remaining Contractual Term (in years)	4 years 1 month 17 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 6.90
Number of Options exercisable (in shares)	120,403
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 6.90
$6.96 to $10.85
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	6.96
Range of Exercise Prices upper range limit (in dollars per share)	$ 10.85
Number of Options outstanding (in shares)	953,089
Weighted Average Remaining Contractual Term (in years)	6 years 10 months 6 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 10.76
Number of Options exercisable (in shares)	604,037
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 10.71
$11.14 to $12.58
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	11.14
Range of Exercise Prices upper range limit (in dollars per share)	$ 12.58
Number of Options outstanding (in shares)	947,839
Weighted Average Remaining Contractual Term (in years)	4 years 8 months 19 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 12.17
Number of Options exercisable (in shares)	887,207
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 12.18
$12.66 to $13.94
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	12.66
Range of Exercise Prices upper range limit (in dollars per share)	$ 13.94
Number of Options outstanding (in shares)	967,420
Weighted Average Remaining Contractual Term (in years)	7 years 3 months 21 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 13.62
Number of Options exercisable (in shares)	455,114
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 13.59
$14.24 to $16.07
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	14.24
Range of Exercise Prices upper range limit (in dollars per share)	$ 16.07
Number of Options outstanding (in shares)	1,142,513
Weighted Average Remaining Contractual Term (in years)	5 years 9 months 3 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 15.31
Number of Options exercisable (in shares)	782,818
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 15.22
$16.09 to $17.16
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	16.09
Range of Exercise Prices upper range limit (in dollars per share)	$ 17.16
Number of Options outstanding (in shares)	1,380,220
Weighted Average Remaining Contractual Term (in years)	7 years 6 months 29 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 16.69
Number of Options exercisable (in shares)	687,691
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 16.57
$17.24 to $22.76
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	17.24
Range of Exercise Prices upper range limit (in dollars per share)	$ 22.76
Number of Options outstanding (in shares)	1,387,909
Weighted Average Remaining Contractual Term (in years)	6 years 6 months 3 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 21.04
Number of Options exercisable (in shares)	1,081,677
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 21.31
$22.84 to $30.46
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	22.84
Range of Exercise Prices upper range limit (in dollars per share)	$ 30.46
Number of Options outstanding (in shares)	1,355,366
Weighted Average Remaining Contractual Term (in years)	8 years 4 months 13 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 28.54
Number of Options exercisable (in shares)	57,644
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 25.41
$30.86 to $32.46
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range
Range of Exercise Prices lower range limit (in dollars per share)	30.86
Range of Exercise Prices upper range limit (in dollars per share)	$ 32.46
Number of Options outstanding (in shares)	166,933
Weighted Average Remaining Contractual Term (in years)	8 years 7 months 6 days
Outstanding Options, Weighted Average Exercise Price (in dollars per share)	$ 31.20
Number of Options exercisable (in shares)	13,069
Exercisable Options, Weighted Average Exercise Price (in dollars per share)	$ 31.57