Fair value measurements - Additional Information (Detail)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) Milestone	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents maturities		Three months or less
Realized gain (loss) on available-for-sale securities		$ 0
Reclassification out of accumulated other comprehensive loss		0
Unrealized Loss on Securities	$ 200,200,000	200,200,000	$ 638,100,000
Unrealized loss on securities, more than twelve months	2,500,000	2,500,000
Investments with other-than-temporary impairment	0	0	0
Contingent consideration, current	2,914,000	2,914,000	3,584,000
Contingent consideration, non current	3,168,000	3,168,000	5,082,000
Pregenen
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent cash payments	129,000,000	129,000,000
Pregenen | Preclinical Milestones [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent payment	$ 5,000,000
Number of milestones | Milestone	2
Contingent cash payments	$ 9,000,000	9,000,000
Pregenen | 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
Pregenen | 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
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aggregate unrealized loss for securities in an unrealized loss position for more than twelve months		$ 100,000	$ 100,000
Milestone achievement period		2026
Milestone discount rates		12.80%
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period		2017
Milestone discount rates		9.80%