Cash and Investments in Marketable Securities - Cash and Investments in Marketable Securities, Including Cash Equivalents (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and cash equivalents	$ 63,295	$ 55,570	$ 18,819	$ 12,365
Short-term investments	190,216	253,374
Total cash and investments in marketable securities	$ 253,511	$ 308,944