Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Billions	Jul. 31, 2018	Jun. 19, 2018	Jul. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Common stock, par value				$ 0.0001	$ 0.0001
Merger Agreement [Member] | Merger Sub [Member]
Subsequent Event [Line Items]
Common stock, par value		$ 0.0001
Offer price, per share		$ 137.00
Merger Agreement [Member] | Merger Sub [Member] | Subsequent Events [Member]
Subsequent Event [Line Items]
Number of shares issued under tender			12,535,376
Percentage of outstanding shares tendered			77.30%
Number of shares not yet tendered	1,342,573
Percentage of shares not yet tendered	3.60%
Aggregate consideration received	$ 2.2
Merger Agreement [Member] | Merger Sub [Member] | Subsequent Events [Member] | Roche [Member]
Subsequent Event [Line Items]
Percentage of outstanding shares tendered including shares held by parent			90.10%