Acquisition of Cascadian - Schedule of Business Acquisition, Unaudited Pro Forma Information (Detail) - Cascadian Therapeutics [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Revenues	$ 169,424	$ 135,291	$ 480,187	$ 352,645
Net income (loss)	$ (67,446)	$ 35,960	$ (131,822)	$ (136,401)
Net income (loss) per share - basic (in dollars per share)	$ (0.42)	$ 0.23	$ (0.83)	$ (0.88)
Net income (loss) per share - diluted (in dollars per share)	$ (0.42)	$ 0.22	$ (0.83)	$ (0.88)