Cash, Cash Equivalents, and Short-Term Investments - Summary of Cost and Estimated Fair Value of Short-Term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Short Term Investments [Abstract]
Amortized Cost of short-term investments, Due within one year	$ 111,172
Amortized Cost of short-term investments, Due within two years	68,338
Short-term investments, Amortized Cost	179,510	$ 154,151
Estimated fair value of short-term investments, Due within one year	111,045
Estimated fair value of short-term investments, Due within two years	68,224
Short-term investments, Estimated Fair Value	$ 179,269	$ 153,749	[1]

[1] The Condensed Consolidated Balance Sheet as of December 31, 2016 has been derived from the audited consolidated financial statements at that date but does not include all of the information and footnotes required by generally accepted accounting principles for complete financial statements.