Acquisitions	3 Months Ended
Mar. 31, 2019
Business Combinations [Abstract]
Acquisitions	Acquisitions Proposed Acquisition of Nightstar Therapeutics plc In March 2019 we entered into an agreement to acquire Nightstar Therapeutics plc (NST), a clinical-stage gene therapy company focused on adeno-associated virus treatments for inherited retinal disorders. NST's lead asset is NSR-REP1 for the potential treatment of choroideremia, a rare, degenerative, X-linked inherited retinal disorder, which leads to blindness and has no approved treatments. NST’s second clinical program is NSR-RPGR for the potential treatment of X-linked retinitis pigmentosa, which is a rare inherited retinal disease with no approved treatments. Under the terms of the proposed acquisition, we would pay NST shareholders $25.50 in cash for each issued and outstanding NST share, which represents an expected total transaction value of approximately $800.0 million on a fully diluted basis, after expected transaction expenses and anticipated cash acquired at closing. We plan to fund the proposed acquisition of NST through available cash and to account for it as an acquisition of a business. It is intended that the proposed acquisition will be implemented by means of a U.K. Court-sanctioned scheme of arrangement under Part 26 of the U.K. Companies Act 2006. The proposed acquisition remains subject to customary closing conditions, including the approval by NST shareholders and the issuance of an order by the U.K. Court. We expect to complete the proposed acquisition by mid-year 2019.

Acquisitions

Proposed Acquisition of Nightstar Therapeutics plc

In March 2019 we entered into an agreement to acquire Nightstar Therapeutics plc (NST), a clinical-stage gene therapy company focused on adeno-associated virus treatments for inherited retinal disorders. NST's lead asset is NSR-REP1 for the potential treatment of choroideremia, a rare, degenerative, X-linked inherited retinal disorder, which leads to blindness and has no approved treatments. NST’s second clinical program is NSR-RPGR for the potential treatment of X-linked retinitis pigmentosa, which is a rare inherited retinal disease with no approved treatments.

Under the terms of the proposed acquisition, we would pay NST shareholders $25.50 in cash for each issued and outstanding NST share, which represents an expected total transaction value of approximately $800.0 million on a fully diluted basis, after expected transaction expenses and anticipated cash acquired at closing. We plan to fund the proposed acquisition of NST through available cash and to account for it as an acquisition of a business.

It is intended that the proposed acquisition will be implemented by means of a U.K. Court-sanctioned scheme of arrangement under Part 26 of the U.K. Companies Act 2006. The proposed acquisition remains subject to customary closing conditions, including the approval by NST shareholders and the issuance of an order by the U.K. Court. We expect to complete the proposed acquisition by mid-year 2019.