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Other investments and investment receivable - Additional Information (Detail) - USD ($) shares in Millions			3 Months Ended		9 Months Ended
	Sep. 28, 2018	Aug. 20, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Research and development expense			$ 116,744,000	$ 61,545,000	$ 328,867,000	$ 180,464,000
Other investment and investment receivable			20,017,000		20,017,000
Gritstone Oncology, Inc. [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Other investment and investment receivable			20,000,000		20,000,000
Gritstone Oncology, Inc. [Member] \| Series C Preferred Shares [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Payments to purchase investments		$ 10,000,000
Gritstone Oncology, Inc. [Member] \| Common Shares [Member] \| Initial Public Offering [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Payments to purchase investments	$ 10,000,000
Number of securities purchased	0.7
Gritstone Oncology, Inc. [Member] \| Other Investment and Investment Receivable [Member] \| Series C Preferred Shares [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Investment recorded at cost			10,000,000		10,000,000
Gritstone Oncology, Inc. [Member] \| Other Investment and Investment Receivable [Member] \| Common Shares [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Investment recorded at cost			10,000,000		10,000,000
License and Collaboration Agreement [Member] \| Gritstone Oncology, Inc. [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Upfront payment		20,000,000
Upfront future payment		27,500,000
Prepaid research and development costs			14,500,000		14,500,000
Research and development expense			5,500,000
License and Collaboration Agreement [Member] \| Gritstone Oncology, Inc. [Member] \| Prepaid Expenses [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Prepaid research and development costs			3,200,000		3,200,000
License and Collaboration Agreement [Member] \| Gritstone Oncology, Inc. [Member] \| Restricted Cash and Other Non-current Assets [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Prepaid research and development costs			$ 11,300,000		$ 11,300,000
License and Collaboration Agreement [Member] \| Gritstone Oncology, Inc. [Member] \| Maximum [Member] \| Therapy Product [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Upfront future payment		$ 129,000,000