Business Combination - Activity - Lumara (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		2 Months Ended	12 Months Ended
	Nov. 12, 2014	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Aug. 31, 2015	Mar. 31, 2015	Nov. 11, 2014
Business Acquisition [Line Items]
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01	$ 0.01
Components of total purchase price:
Fair value of AMAG common stock issued				$ 111,964
Fair value of contingent milestone payments		$ 205,000		205,000	$ 13,700
Fair values assigned to assets acquired and the liabilities assumed
Goodwill		205,824	$ 639,188	205,824
Other disclosures
Share price (in dollars per share)						$ 63.75	$ 44.00	$ 34.88
Net deferred tax liability		77,619	189,145	77,619
Acquisition-related costs			11,232	9,478	$ 782
Lumara Health
Business Acquisition [Line Items]
Ownership percentage acquired	100.00%
Additional merger consideration based upon achievement of certain net sales milestones
Future contingent payments, maximum	$ 350,000
Maximum amount of contingent payments due in any calendar year				100,000
Components of total purchase price:
Cash consideration	600,000
Fair value of AMAG common stock issued	111,964
Fair value of contingent milestone payments	205,000
Estimated working capital and other adjustments	821
Purchase price paid at closing	917,785
Less: Cash received on finalization of the net working capital and other adjustments	(562)
Cash acquired from Lumara Health	(5,219)
Total purchase price	912,004
Fair values assigned to assets acquired and the liabilities assumed
Accounts receivable	36,852
Inventories	30,300
Prepaid and other current assets	3,322
Deferred income tax assets	102,355
Property and equipment	60
Restricted cash - short-term	1,997
Other long-term assets	3,412
Accounts payable	(3,807)
Accrued expenses	(36,561)
Deferred income tax liabilities	(295,676)
Other long-term liabilities	(4,563)
Total estimated identifiable net assets	713,891
Goodwill	198,113		198,100
Total	912,004
Gross accounts receivable	$ 40,500
Other disclosures
Discount rate (as a percent)	5.00%
Gross accounts receivable	$ 40,500
Inventory, fair value step-up adjustment	26,100		12,000
2016			4,800
2017			4,000
2018			3,200
Net deferred tax liability			$ 193,300
Combined federal and state statutory income tax rate (as a percent)			38.80%
Acquisition-related costs				9,500
Revenue generated after the acquisition date		$ 22,500
Lumara Health | Cost of Sales
Other disclosures
Amortization of fair value adjustment			$ 11,600	1,300
Lumara Health | Research and Development Expense
Other disclosures
Amortization of fair value adjustment			1,200
Lumara Health | Makena IPR&D
Fair values assigned to assets acquired and the liabilities assumed
Indefinite-lived intangible assets	79,100
Lumara Health | Makena Marketed Product
Fair values assigned to assets acquired and the liabilities assumed
Finite-lived intangible assets	797,100
First Milestone | Lumara Health
Additional merger consideration based upon achievement of certain net sales milestones
Milestone payment to be paid by company upon milestone achievement				100,000
Aggregate net sales milestone				$ 300,000
Consecutive time period for achievement of milestone				12 months
Second Milestone | Lumara Health
Additional merger consideration based upon achievement of certain net sales milestones
Milestone payment to be paid by company upon milestone achievement				$ 100,000
Aggregate net sales milestone				$ 400,000
Consecutive time period for achievement of milestone				12 months
Setoff amount upon which milestone payment is subject to under certain conditions				$ 50,000
Third Milestone | Lumara Health
Additional merger consideration based upon achievement of certain net sales milestones
Milestone payment to be paid by company upon milestone achievement				50,000
Aggregate net sales milestone				$ 700,000
Consecutive time period for achievement of milestone				24 months
Setoff amount upon which milestone payment is subject to under certain conditions				$ 100,000
Fourth Milestone | Lumara Health
Additional merger consideration based upon achievement of certain net sales milestones
Milestone payment to be paid by company upon milestone achievement				100,000
Aggregate net sales milestone				$ 500,000
Consecutive time period for achievement of milestone				12 months
Fifth Milestone [Member] | Lumara Health
Additional merger consideration based upon achievement of certain net sales milestones
Milestone payment to be paid by company upon milestone achievement				$ 50,000
Aggregate net sales milestone				$ 200,000
Consecutive time period for achievement of milestone				5 years
Indemnification Escrow Fund | Lumara Health
Other disclosures
Escrow fund	$ 35,000
Common Stock
Business Acquisition [Line Items]
Shares of AMAG common stock issued in business combination				3,210
Components of total purchase price:
Fair value of AMAG common stock issued				$ 32
Common Stock | Lumara Health
Business Acquisition [Line Items]
Shares of AMAG common stock issued in business combination	3,200
Early Adoption | ASU 2015-16 | Lumara Health
Measurement period adjustments
Revenue reserves adjustment			7,200
Reduction in deferred tax liabilities			5,400
Final settlement adjustment			$ 4,500