Fair value measurements - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities			Three months or less
Realized gain on available-for-sale securities			$ 0
Reclassification out of accumulated other comprehensive loss			0
Unrealized Loss on Securities	$ 481,800,000		481,800,000		$ 376,100,000
Unrealized loss on securities, more than twelve months	15,600,000		15,600,000		95,500,000
Investments with other-than-temporary impairment	0		0		0
Achievement of research milestone	(970,000)	$ 1,404,000	463,000	$ 2,417,000
Contingent consideration, non current	3,219,000		3,219,000		$ 3,277,000
Pregenen [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	120,000,000		120,000,000
Pregenen [Member] | Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	99,900,000		99,900,000
Pregenen [Member] | Preclinical Milestones [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Achievement of research milestone	5,000,000
Pregenen [Member] | Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	$ 20,100,000		$ 20,100,000
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2019
Milestone discount rates			14.20%
Minimum [Member] | Pregenen [Member] | Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2021
Minimum [Member] | Pregenen [Member] | Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2021
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2026
Milestone discount rates			15.50%
Maximum [Member] | Pregenen [Member] | Commercial Milestones Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2028
Maximum [Member] | Pregenen [Member] | Clinical Milestone Payments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period			2028