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License and royalty revenue - Additional Information (Detail)			1 Months Ended	3 Months Ended			6 Months Ended	9 Months Ended
	Apr. 28, 2017 USD ($) PerformanceObligation	Apr. 26, 2017 USD ($) PerformanceObligation	Aug. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
License And Royalty Revenue [Line Items]
Revenue				$ 11,528,000	$ 7,711,000			$ 35,336,000	$ 31,259,000
Cost of license and royalty revenue				29,000	1,100,000			67,000	1,520,000
License and Royalty [Member]
License And Royalty Revenue [Line Items]
Revenue				602,000	2,500,000			1,365,000	13,070,000
Novartis Pharma AG [Member]
License And Royalty Revenue [Line Items]
License agreement upfront payment		$ 7,500,000
Revenue recognized upon achievement of a related milestone			$ 2,500,000
Novartis Pharma AG [Member] \| Royalty [Member]
License And Royalty Revenue [Line Items]
Revenue				600,000		$ 0	$ 0	1,400,000
Novartis Pharma AG [Member] \| License and Royalty [Member]
License And Royalty Revenue [Line Items]
Cost of license and royalty revenue					1,100,000				1,400,000
Novartis Pharma AG [Member] \| License and Royalty [Member] \| Maximum [Member]
License And Royalty Revenue [Line Items]
Cost of license and royalty revenue				100,000				100,000
Novartis Pharma AG [Member] \| Topic 606 [Member]
License And Royalty Revenue [Line Items]
Number of performance obligation identified at the date of contract inception \| PerformanceObligation		1
Revenue						7,500,000
Novartis Pharma AG [Member] \| Topic 606 [Member] \| Regulatory Milestones Payments [Member]
License And Royalty Revenue [Line Items]
Revenue						2,500,000
Orchard Therapeutics Limited [Member]
License And Royalty Revenue [Line Items]
License agreement upfront payment	$ 3,000,000
Orchard Therapeutics Limited [Member] \| License and Royalty [Member]
License And Royalty Revenue [Line Items]
Revenue				0				0
Cost of license and royalty revenue				0	100,000			0	100,000
Orchard Therapeutics Limited [Member] \| Topic 606 [Member]
License And Royalty Revenue [Line Items]
Number of performance obligation identified at the date of contract inception \| PerformanceObligation	1
Revenue						$ 3,000,000
Number of unsatisfied performance obligation identified at date of contract inception \| PerformanceObligation	0
Orchard Therapeutics Limited [Member] \| Topic 606 [Member] \| License [Member]
License And Royalty Revenue [Line Items]
Revenue				$ 0	$ 3,000,000			$ 0	$ 3,000,000
Regulatory Milestones Payments [Member] \| Novartis Pharma AG [Member]
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 7,500,000
Each Subsequently Licensed Product [Member] \| Novartis Pharma AG [Member]
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 1,100,000
Potential Milestones Payments [Member] \| Orchard Therapeutics Limited [Member]
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event	$ 1,300,000