Business (Details) $ / shares in Units, $ in Thousands, shares in Millions	Mar. 31, 2019 USD ($) program $ / shares shares	Dec. 31, 2018 USD ($) $ / shares
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of gene therapy programs \| program	14
Exchange agreement, aggregate principal amount	$ 219,300
Shares authorized for issuance (in shares) \| shares	39.0
Common stock, par value (in dollars per share) \| $ / shares	$ 0.01	$ 0.01
Accumulated deficit	$ (1,532,521)	$ (1,412,222)

The cumulative amount of the reporting entity's undistributed earnings or deficit.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(a)(4))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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