Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Significant Accounting Policies [Line Items]
Realized gain on sale of marketable securities	$ 6,977		$ 2,081
Marketable securities classified as cash equivalents, maximum original maturity	90 days
Policy for marketable securities	90 days
Deferred revenue, current	$ 33,540	$ 15,352
Deferred revenue, net of current portion	49,392	52,965
Research and Development
Significant Accounting Policies [Line Items]
License fee	2,400	$ 3,500	$ 12,100
Regulus Therapeutics Inc
Significant Accounting Policies [Line Items]
Realized gain on sale of marketable securities	$ 7,000