Cash and Investments in Marketable Securities - Cash and Investments in Marketable Securities, Including Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and cash equivalents	$ 18,819	$ 12,365	$ 32,008	$ 39,067
Short-term investments	249,000	225,459
Restricted cash	0	25,000
Total cash and investments in marketable securities	$ 267,819	$ 262,824