Subsequent Events	12 Months Ended
Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On February 20, 2020, the Company entered into the Amended Merger Agreement with Progenics, which amends and restates the Initial Merger Agreement. Under the terms of the Amended Merger Agreement, the Company will acquire all of the issued and outstanding shares of Progenics common stock at a fixed exchange ratio whereby Progenics stockholders will receive, for each share of Progenics stock held at the time of the closing of the merger, 0.31 of a share of the Company’s common stock, increased from 0.2502 under the Initial Merger Agreement, together with a non-tradeable contingent value right (a “CVR”) tied to the financial performance of PyL^TM (¹⁸F-DCFPyL), Progenics’ prostate-specific membrane antigen targeted imaging agent designed to visualize prostate cancer currently in late stage clinical development (“PyL”). Each CVR will entitle its holder to receive a pro rata share of aggregate cash payments equal to 40% of U.S. net sales generated by PyL in 2022 and 2023 in excess of $100 million and $150 million, respectively. In no event will the Company’s aggregate payments under the CVRs exceed 19.9% of the total consideration the Company pays in the transaction. As a result of the increase in the exchange ratio, following the completion of the merger, former Progenics stockholders’ aggregate ownership stake will increase to approximately 40% of the combined company from approximately 35% under the Initial Merger Agreement. Progenics’ stockholders will also now be entitled to appraisal rights as provided under Delaware law. The transaction contemplated by the Amended Merger Agreement was unanimously approved by the Boards of Directors of both companies and requires, among other things, the affirmative vote of a majority of the outstanding shares of common stock of Progenics and a majority of votes cast by the holders of the common stock of the Company. In addition, pursuant to the Amended Merger Agreement, the holder of each in-the-money option to purchase shares of Progenics common stock under any equity based compensation plan of Progenics (“Progenics Stock Option”) will be entitled to receive in exchange for each such in-the-money option (i) an option to purchase Lantheus Common Stock (each, a “Lantheus Stock Option”) converted based on the 0.31 exchange ratio and (ii) a vested or unvested CVR depending on whether the underlying option is vested. Holders of out-of-the-money Progenics Stock Options will receive Lantheus Stock Options converted on an exchange ratio adjusted based on actual trading prices of common stock of Progenics and Lantheus Holdings prior to the effective time of the merger. The Amended Merger Agreement also provides that on closing the Company’s board of directors will appoint Dr. Gerard Ber and Mr. Heinz Mausli, who are currently members of the board of directors of Progenics, to serve on the Company’s board of directors. In addition, the Company’s board of directors, subject to complying with applicable fiduciary duties, will use commercially reasonable efforts to cause Dr. Ber and Mr. Mausli to be nominated for reelection following the closing through 2023. The Company’s board of directors will be reduced in size from ten to nine members at the Company’s annual meeting of stockholders on April 23, 2020 (or sooner if the transaction closes before then) and will be further reduced in size from nine to eight members prior to the date of the Company’s 2021 annual meeting of stockholders. Except as described above, the material terms of the Amended Merger Agreement are substantially the same as the terms of the Initial Merger Agreement. The transaction is currently expected to close in the second quarter of 2020. Upon completion of the acquisition, which the parties intend to report as tax-deferred to Progenics’ stockholders with respect to the stock component of the merger consideration for U.S. federal income tax purposes, the combined company will continue to be headquartered in North Billerica, Massachusetts and will trade on the NASDAQ under the ticker symbol LNTH.

Subsequent Events

On February 20, 2020, the Company entered into the Amended Merger Agreement with Progenics, which amends and restates the Initial Merger Agreement. Under the terms of the Amended Merger Agreement, the Company will acquire all of the issued and outstanding shares of Progenics common stock at a fixed exchange ratio whereby Progenics stockholders will receive, for each share of Progenics stock held at the time of the closing of the merger, 0.31 of a share of the Company’s common stock, increased from 0.2502 under the Initial Merger Agreement, together with a non-tradeable contingent value right (a “CVR”) tied to the financial performance of PyL^TM (¹⁸F-DCFPyL), Progenics’ prostate-specific membrane antigen targeted imaging agent designed to visualize prostate cancer currently in late stage clinical development (“PyL”). Each CVR will entitle its holder to receive a pro rata share of aggregate cash payments equal to 40% of U.S. net sales generated by PyL in 2022 and 2023 in excess of $100 million and $150 million, respectively. In no event will the Company’s aggregate payments under the CVRs exceed 19.9% of the total consideration the Company pays in the transaction. As a result of the increase in the exchange ratio, following the completion of the merger, former Progenics stockholders’ aggregate ownership stake will increase to approximately 40% of the combined company from approximately 35% under the Initial Merger Agreement. Progenics’ stockholders will also now be entitled to appraisal rights as provided under Delaware law. The transaction contemplated by the Amended Merger Agreement was unanimously approved by the Boards of Directors of both companies and requires, among other things, the affirmative vote of a majority of the outstanding shares of common stock of Progenics and a majority of votes cast by the holders of the common stock of the Company.

In addition, pursuant to the Amended Merger Agreement, the holder of each in-the-money option to purchase shares of Progenics common stock under any equity based compensation plan of Progenics (“Progenics Stock Option”) will be entitled to receive in exchange for each such in-the-money option (i) an option to purchase Lantheus Common Stock (each, a “Lantheus Stock Option”) converted based on the 0.31 exchange ratio and (ii) a vested or unvested CVR depending on whether the underlying option is vested. Holders of out-of-the-money Progenics Stock Options will receive Lantheus Stock Options converted on an exchange ratio adjusted based on actual trading prices of common stock of Progenics and Lantheus Holdings prior to the effective time of the merger.

The Amended Merger Agreement also provides that on closing the Company’s board of directors will appoint Dr. Gerard Ber and Mr. Heinz Mausli, who are currently members of the board of directors of Progenics, to serve on the Company’s board of directors. In addition, the Company’s board of directors, subject to complying with applicable fiduciary duties, will use commercially reasonable efforts to cause Dr. Ber and Mr. Mausli to be nominated for reelection following the closing through 2023. The Company’s board of directors will be reduced in size from ten to nine members at the Company’s annual meeting of stockholders on April 23, 2020 (or sooner if the transaction closes before then) and will be further reduced in size from nine to eight members prior to the date of the Company’s 2021 annual meeting of stockholders.

Except as described above, the material terms of the Amended Merger Agreement are substantially the same as the terms of the Initial Merger Agreement.

The transaction is currently expected to close in the second quarter of 2020. Upon completion of the acquisition, which the parties intend to report as tax-deferred to Progenics’ stockholders with respect to the stock component of the merger consideration for U.S. federal income tax purposes, the combined company will continue to be headquartered in North Billerica, Massachusetts and will trade on the NASDAQ under the ticker symbol LNTH.