Marketable Securities - Summary of Cash Equivalents and Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	$ 49,826	$ 22,061	$ 69,482	$ 6,030
Amortized Cost	223,634	139,416
Gross Unrealized Gains		76
Gross Unrealized (Losses)	(255)	(26)
Estimated Fair Value	223,379	139,466
Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	194,749	120,424
Gross Unrealized Gains		76
Gross Unrealized (Losses)	(255)	(26)
Estimated Fair Value	194,494	120,474
Money market funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	24,290	18,992
Total cash equivalents	24,290	18,992
Commercial paper securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	4,595
Total cash equivalents	4,595
Commercial paper securities [Member] | Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	110,365	23,112
Gross Unrealized Gains		73
Gross Unrealized (Losses)	(118)
Estimated Fair Value	110,247	23,185
Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	28,885	18,992
Total cash equivalents	28,885	18,992
Corporate debt securities [Member] | Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	75,886	10,005
Gross Unrealized (Losses)	(131)	(1)
Estimated Fair Value	75,755	10,004
U.S. government sponsored entity debt securities [Member] | Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	8,498	87,307
Gross Unrealized Gains		3
Gross Unrealized (Losses)	(6)	(25)
Estimated Fair Value	$ 8,492	$ 87,285