Business Combination - Activity (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions			3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 17, 2015	Nov. 12, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Business Acquisition [Line Items]
Cash received on finalization of the net working capital and other adjustments					$ (562)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Goodwill			$ 637,400		$ 637,400		$ 205,824
Combined federal and state statutory income tax rate (as a percent)					37.00%
Net deferred tax liability			144,900		$ 144,900
Acquisition-related costs			$ 8,500	$ 1,917	$ 11,153	$ 1,917
Common stock, par value (in dollars per share)			$ 0.01		$ 0.01		$ 0.01
Future contingent payments, maximum		$ 350,000
CBR Acquisition Holdings Corp
Business Acquisition [Line Items]
Cash consideration	$ 700,000
Estimated working capital and other adjustments	(17,837)
Purchase price paid at closing	682,163
Plus: Estimated purchase price payable to sellers	188
Total purchase price paid	682,351
Excess of payable to former shareholders	7,200
Net working capital and other adjustment			$ 17,600
Increase in cash consideration			200
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Accounts receivable	9,509
Inventories	4,443
Prepaid and other current assets	8,487
Restricted cash	30,752
Property, plant and equipment	29,669
Deferred income tax assets	4,555
Other long-term assets	198
Accounts payable	(2,853)
Accrued expenses	(13,575)
Deferred revenues	(3,100)
Payable to former CBR shareholders	(37,947)
Deferred income tax liabilities	(149,497)
Total estimated identifiable net assets	243,064
Goodwill	439,287		439,300		$ 439,300
Total	682,351
Contractual amount of accounts receivable	$ 11,600
Useful life	20 years				20 years
Net deferred tax liability			149,500		$ 149,500
Acquisition-related costs			8,500		11,200
Revenue generated after the acquisition date			7,200		7,200
CBR Acquisition Holdings Corp | CBR Customer Relationships
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Finite-lived intangible assets	$ 297,000
CBR Acquisition Holdings Corp | CBR Trade Names and Trademarks
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Indefinite-lived intangible assets	65,000
CBR Acquisition Holdings Corp | Favorable Lease
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Finite-lived intangible assets	$ 423
Lumara Health
Business Acquisition [Line Items]
Cash consideration		600,000
Fair value of AMAG common stock isued		111,964
Fair value of contingent milestone payments		205,000
Estimated working capital and other adjustments		821
Purchase price paid at closing		917,785
Cash received on finalization of the net working capital and other adjustments		(562)
Cash acquired from Lumara Health		(5,219)
Total purchase price paid		$ 912,004
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Accounts receivable			36,852		36,852
Inventories			30,300		30,300
Prepaid and other current assets			3,322		3,322
Restricted cash			1,997		1,997
Property, plant and equipment			60		60
Finite-lived intangible assets			797,100		797,100
Deferred income tax assets			102,355		102,355
Other long-term assets			3,412		3,412
Accounts payable			(3,807)		(3,807)
Accrued expenses			(36,561)		(36,561)
Deferred income tax liabilities			(295,676)		(295,676)
Other long-term liabilities			(4,563)		(4,563)
Total estimated identifiable net assets			713,891		713,891
Goodwill			198,113		198,113		$ 205,800
Total			912,004		$ 912,004
Useful life					20 years
Acquisition-related costs							$ 1,900
Ownership percentage acquired		100.00%
Final settlement adjustment					$ 4,500
Lumara Health | Makena IPR&D
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Indefinite-lived intangible assets			$ 79,100		$ 79,100
Common Stock | Lumara Health
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Shares of AMAG common stock issued in business combination		3.2
Common stock, par value (in dollars per share)		$ 0.01