Subsequent Events	12 Months Ended
Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTS Acquisition of Perosphere Pharmaceuticals Inc. On January 16, 2019, we acquired Perosphere through the merger of our wholly-owned subsidiary, Magellan Merger Sub, Inc., a Delaware corporation (“Merger Sub”), with and into Perosphere, with Perosphere continuing as the surviving entity and our wholly-owned subsidiary (the “Merger”). The acquisition enhances our development pipeline by adding an innovative clinical asset to our portfolio and leveraging our expertise in hematology. As a result of the acquisition of Perosphere, we acquired the global rights to ciraparantag, an anticoagulant reversal agent, which is being investigated for patients treated with novel oral anticoagulants or low molecular weight heparin when reversal of the anticoagulant effect of these products is needed for emergency surgery, urgent procedures or due to life-threatening or uncontrolled bleeding. In addition, provided certain clinical milestones are met, the Phase 3 program for ciraparantag will be partially funded under an existing clinical trial collaboration agreement, as amended, with a global pharmaceutical company, under which we may receive certain payments anticipated in 2019 and 2020 related to ciraparantag for use as an anticoagulant reversal agent to reverse the effects of Savaysa^®(edoxaban) and low molecular weight heparin. The Merger was completed pursuant to the Agreement and Plan of Merger (the “Perosphere Agreement”), dated as of December 12, 2018, by and among, inter alia, AMAG and Perosphere. Pursuant to the Perosphere Agreement, we paid, at closing, an upfront purchase price (the “Upfront Merger Consideration”) of approximately $50.0 million, approximately $40.0 million of which was funded from our available cash and approximately $10.0 million of which was deemed paid in connection with the cancellation of a convertible note in the principal amount of $10.0 million issued to us by Perosphere in October 2018. The purchase price is subject to customary post-closing adjustments under the Perosphere Agreement. In addition to the Upfront Merger Consideration, AMAG used available cash to repay $12.0 million of Perosphere’s term loan indebtedness and assumed approximately $6.2 million of Perosphere’s other liabilities. Under and subject to the terms and conditions set forth in the Perosphere Agreement, we are obligated to pay future contingent consideration of up to an aggregate of $365.0 million (the “Milestone Payments”), including (a) up to an aggregate of $140.0 million that becomes payable conditioned upon the achievement of specified regulatory milestones for ciraparantag (the “Regulatory Milestone Payments”), including a $40.0 million milestone payment conditioned upon approval by the European Medicines Agency and (b) up to an aggregate of $225.0 million that becomes payable conditioned upon the achievement of specified sales milestones (the “Sales Milestone Payments”). If the final label approved for ciraparantag in the U.S. includes a boxed warning, the Regulatory Milestone Payments shall no longer be payable, and any previously paid Regulatory Milestone Payments shall be credited against 50% of any future Milestone Payment that otherwise becomes payable. The first Sales Milestone Payment of $20.0 million will be payable conditioned upon annual net sales of ciraparantag of at least $100.0 million. We are unable to provide preliminary estimates of asset and liability values as the valuation of the assets acquired and liabilities assumed is in progress. 2019 Restructuring In February 2019, we completed a restructuring to combine our women’s health and maternal health sales forces into one integrated sales team, which will promote both Intrarosa and Makena. Approximately 110 employees were displaced through this workforce reduction. We expect to record a one-time restructuring charge of approximately $6.0 million primarily related to severance and related benefits in the first quarter of 2019 and expect the activities to be completed by the end of the first quarter of 2019.

SUBSEQUENT EVENTS

Acquisition of Perosphere Pharmaceuticals Inc.

On January 16, 2019, we acquired Perosphere through the merger of our wholly-owned subsidiary, Magellan Merger Sub, Inc., a Delaware corporation (“Merger Sub”), with and into Perosphere, with Perosphere continuing as the surviving entity and our wholly-owned subsidiary (the “Merger”). The acquisition enhances our development pipeline by adding an innovative clinical asset to our portfolio and leveraging our expertise in hematology.

As a result of the acquisition of Perosphere, we acquired the global rights to ciraparantag, an anticoagulant reversal agent, which is being investigated for patients treated with novel oral anticoagulants or low molecular weight heparin when reversal of the anticoagulant effect of these products is needed for emergency surgery, urgent procedures or due to life-threatening or uncontrolled bleeding. In addition, provided certain clinical milestones are met, the Phase 3 program for ciraparantag will be partially funded under an existing clinical trial collaboration agreement, as amended, with a global pharmaceutical company, under which we may receive certain payments anticipated in 2019 and 2020 related to ciraparantag for use as an anticoagulant reversal agent to reverse the effects of Savaysa^®(edoxaban) and low molecular weight heparin.

The Merger was completed pursuant to the Agreement and Plan of Merger (the “Perosphere Agreement”), dated as of December 12, 2018, by and among, inter alia, AMAG and Perosphere. Pursuant to the Perosphere Agreement, we paid, at closing, an upfront purchase price (the “Upfront Merger Consideration”) of approximately $50.0 million, approximately $40.0 million of which was funded from our available cash and approximately $10.0 million of which was deemed paid in connection with the cancellation of a convertible note in the principal amount of $10.0 million issued to us by Perosphere in October 2018. The purchase price is subject to customary post-closing adjustments under the Perosphere Agreement. In addition to the Upfront Merger Consideration, AMAG used available cash to repay $12.0 million of Perosphere’s term loan indebtedness and assumed approximately $6.2 million of Perosphere’s other liabilities.

Under and subject to the terms and conditions set forth in the Perosphere Agreement, we are obligated to pay future contingent consideration of up to an aggregate of $365.0 million (the “Milestone Payments”), including (a) up to an aggregate of $140.0 million that becomes payable conditioned upon the achievement of specified regulatory milestones for ciraparantag (the “Regulatory Milestone Payments”), including a $40.0 million milestone payment conditioned upon approval by the European Medicines Agency and (b) up to an aggregate of $225.0 million that becomes payable conditioned upon the achievement of specified sales milestones (the “Sales Milestone Payments”). If the final label approved for ciraparantag in the U.S. includes a boxed warning, the Regulatory Milestone Payments shall no longer be payable, and any previously paid Regulatory Milestone Payments shall be credited against 50% of any future Milestone Payment that otherwise becomes payable. The first Sales Milestone Payment of $20.0 million will be payable conditioned upon annual net sales of ciraparantag of at least $100.0 million.

We are unable to provide preliminary estimates of asset and liability values as the valuation of the assets acquired and liabilities assumed is in progress.

2019 Restructuring

In February 2019, we completed a restructuring to combine our women’s health and maternal health sales forces into one integrated sales team, which will promote both Intrarosa and Makena. Approximately 110 employees were displaced through this workforce reduction. We expect to record a one-time restructuring charge of approximately $6.0 million primarily related to severance and related benefits in the first quarter of 2019 and expect the activities to be completed by the end of the first quarter of 2019.