Cash, Cash Equivalents, and Short-Term Investments - Summary of Cost and Estimated Fair Value of Short-Term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Short Term Investments [Abstract]
Amortized Cost of short-term investments, Due in 2017	$ 84,406
Amortized Cost of short-term investments, Due in 2018	69,745
Short-term investments, Amortized Cost	154,151	$ 96,853
Estimated fair value of short-term investments, Due in 2017	84,315
Estimated fair value of short-term investments, Due in 2018	69,434
Short-term investments, Estimated Fair Value	$ 153,749	$ 96,555