Market Capitalization

No definition available.

Name:	amag_MarketCapitalization
Namespace Prefix:	amag_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Net Assets, Percentage Of Fair Value In Excess Of Carrying Amount

+ References

No definition available.

+ Details

Name:	amag_NetAssetsPercentageOfFairValueInExcessOfCarryingAmount
Namespace Prefix:	amag_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Reporting Unit, Carrying Amount

+ References

No definition available.

+ Details

Name:	amag_ReportingUnitCarryingAmount
Namespace Prefix:	amag_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reporting Unit, Measurement Input

+ References

No definition available.

+ Details

Name:	amag_ReportingUnitMeasurementInput
Namespace Prefix:	amag_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Reporting Unit, Measurement Input, Median

+ References

No definition available.

+ Details

Name:	amag_ReportingUnitMeasurementInputMedian
Namespace Prefix:	amag_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Reporting Unit, Percentage Of Fair Value Below Carrying Amount

+ References

No definition available.

+ Details

Name:	amag_ReportingUnitPercentageOfFairValueBelowCarryingAmount
Namespace Prefix:	amag_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Share Price, Median

+ References

No definition available.

+ Details

Name:	amag_SharePriceMedian
Namespace Prefix:	amag_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

+ Details

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

- Definition

Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

+ Details

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

- Definition

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13854-109267

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=108376223&loc=d3e13816-109267

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13777-109266

+ Details

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

- Definition

The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275

+ Details

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

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputControlPremiumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

GOODWILL AND INTANGIBLE ASSETS, NET - Goodwill - Narrative (Details)					2 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Oct. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Nov. 05, 2018 USD ($)	Oct. 31, 2018 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018
Goodwill [Line Items]
Goodwill		$ 422,513,000			$ 422,513,000	$ 422,513,000		$ 422,513,000	$ 422,513,000
Accumulated impairment losses						0		0
Share price (in dollars per share) \| $ / shares	$ 9.71	$ 15.19		$ 21.50	$ 15.19				$ 15.19
Market capitalization	$ 329,000,000		$ 633,000,000	$ 742,000,000	$ 617,000,000
Fair value percentage below carrying amount, reporting unit	6.00%	41.00%	20.00%	2.00%	21.00%
Fair value percentage above carrying amount, net assets	29.00%	15.00%		36.00%
Goodwill, impairment	$ 0			$ 0			$ 0	$ 0	$ 0
Share price, median (in dollars per share) \| $ / shares					$ 17.84
Reporting unit, carrying amount		$ 747,000,000			$ 747,000,000				$ 747,000,000
Impairment of intangible assets						$ 151,300,000
Measurement Input, Control Premium
Goodwill [Line Items]
Reporting unit, measurement input, median						0.65		0.65		0.71
Measurement Input, Control Premium \| Minimum
Goodwill [Line Items]
Reporting unit, measurement input						0.37		0.37		0.39
Measurement Input, Control Premium \| Maximum
Goodwill [Line Items]
Reporting unit, measurement input						0.84		0.84		0.96