Marketable Securities - Schedule of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	$ 30,708	$ 49,826
Amortized Cost	224,300	223,634
Gross Unrealized Gains	8
Gross Unrealized (Losses)	(362)	(255)
Estimated Fair Value	223,946	223,379
Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	204,158	194,749
Gross Unrealized Gains	8
Gross Unrealized (Losses)	(362)	(255)
Estimated Fair Value	203,804	194,494
Money market funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	20,142	24,290
Total cash equivalents	20,142	24,290
Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Cash and cash equivalents	20,142	28,885
Total cash equivalents	20,142	28,885
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	126,033	110,365
Gross Unrealized Gains	8
Gross Unrealized (Losses)	(160)	(118)
Estimated Fair Value	125,881	110,247
Corporate debt securities [Member] | Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	75,125	75,886
Gross Unrealized (Losses)	(202)	(131)
Estimated Fair Value	74,923	75,755
U.S. government sponsored entity debt securities [Member] | Available-for-sale securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	3,000	8,498
Gross Unrealized (Losses)		(6)
Estimated Fair Value	$ 3,000	$ 8,492