Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands					1 Months Ended
	Jan. 22, 2018	Jan. 08, 2018	Jun. 26, 2017	Mar. 21, 2014	Jan. 31, 2018
Public Offering
Shares issued (in shares)			4,111,250
Share price (in dollars per share)			$ 70.00
Net proceeds from issuance of common stock			$ 269,900
Regenx Bio SMA License | ReGenX | Maximum
License agreements
Potential milestone payments to be paid				$ 12,250
Subsequent Event [Member]
Public Offering
Shares issued (in shares)	4,509,840
Share price (in dollars per share)	$ 102.00
Net proceeds from issuance of common stock	$ 431,900				$ 431,900
Subsequent Event [Member] | Regenx Bio SMA License | ReGenX
License agreements
Up-front payment, paid		$ 80,000
Subsequent Event [Member] | First Anniversary Of Amendment | Regenx Bio SMA License | ReGenX
License agreements
Potential milestone payments to be paid		30,000
Subsequent Event [Member] | Second Anniversary Of Amendment | Regenx Bio SMA License | ReGenX
License agreements
Potential milestone payments to be paid		30,000
Subsequent Event [Member] | Achievement Of Sales Based Milestones | Regenx Bio SMA License | ReGenX | Maximum
License agreements
Potential milestone payments to be paid		$ 120,000
Over-Allotment Option
Public Offering
Shares issued (in shares)			536,250
Over-Allotment Option | Subsequent Event [Member]
Public Offering
Shares issued (in shares)	588,240