Fair value measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
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	$ 0
Reclassification out of accumulated other comprehensive income (loss)	0	0
Unrealized Loss on Securities	376,100,000	638,100,000
Unrealized loss on securities, more than twelve months	95,500,000	0
Investments with other-than-temporary impairment	0	0
Contingent consideration, current	4,479,000	3,584,000
Contingent consideration, non current	3,277,000	5,082,000
Pregenen [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent payment	$ 5,000,000	$ 1,000,000
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2017
Milestone discount rates	10.10%
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone achievement period	2026
Milestone discount rates	13.10%