Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	1 Months Ended		3 Months Ended
	Apr. 30, 2018	Apr. 30, 2018	Mar. 31, 2017
Subsequent Event [Line Items]
Net proceeds from sale of shares, after deducting underwriting discounts and commissions and other estimated offering expenses			$ 3,425
Subsequent Event [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Sale of common stock in underwritten public offering	14.2
Sale of common stock, public offering price	$ 16.25	$ 16.25
Net proceeds from sale of shares, after deducting underwriting discounts and commissions and other estimated offering expenses	$ 215,900
Kite Pharma, Inc. ("Kite") [Member] | Collaboration and License Agreement [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Milestone payments received		$ 150,000