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Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 22, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Cash and cash equivalents		$ 88,419		$ 88,419		$ 61,729
Accumulated deficit		(668,379)		(668,379)		$ (566,329)
Research and development expense		$ 8,641	$ 8,263	$ 28,115	$ 25,935
Subsequent Event [Member] \| Two Thousand Nineteen Agreement [Member]
Number of shares issued for warrants	3,333,333
Exercise price of warrants	$ 0.001
License agreement termination date	Dec. 31, 2026
Warrants expire date	Dec. 31, 2026
Subsequent Event [Member] \| MD Anderson License and the Research and Development Agreement Member [Member] \| Two Thousand Nineteen Agreement [Member]
Research and development expense	$ 20,000
Subsequent Event [Member] \| MD Anderson License and the Research and Development Agreement Member [Member] \| Two Thousand Nineteen Agreement [Member] \| Performance Based Payments Member [Member]
Aggregate Benchmark Payments Payable	36,500
Subsequent Event [Member] \| MD Anderson License and the Research and Development Agreement Member [Member] \| Two Thousand Nineteen Agreement [Member] \| Performance Based Payments Member [Member] \| Post Marketing Approval [Member]
Aggregate Benchmark Payments Payable	$ 3,000