Marketable Securities and Fair Value Measurements - Short-term and Long-term Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 272,280	$ 251,086
Gross Unrealized Gains	3	38
Gross Unrealized Losses	(252)	(143)
Fair Value	272,031	250,981
Short-term Investments \| Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,503	42,397
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(6)
Fair Value	58,502	42,391
Short-term Investments \| Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	145,728	122,788
Gross Unrealized Gains	3	22
Gross Unrealized Losses	(174)	(121)
Fair Value	145,557	122,689
Short-term Investments \| Municipal Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		5,852
Gross Unrealized Gains		0
Gross Unrealized Losses		(5)
Fair Value		5,847
Short-term Investments \| U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,013	28,903
Gross Unrealized Gains	0	9
Gross Unrealized Losses	(7)	(4)
Fair Value	3,006	28,908
Short-term Investments \| U.S. Government Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,650	45,146
Gross Unrealized Gains	0	7
Gross Unrealized Losses	(70)	(7)
Fair Value	60,580	45,146
Short-term Investments \| Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,386	6,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,386	6,000
Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,090	59,887
Gross Unrealized Gains	2	13
Gross Unrealized Losses	(144)	(117)
Fair Value	39,948	59,783
Long Term Investments \| Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,067	40,889
Gross Unrealized Gains	2	8
Gross Unrealized Losses	(76)	(85)
Fair Value	24,993	40,812
Long Term Investments \| U.S. government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,023	6,805
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(68)	(16)
Fair Value	$ 14,955	6,789
Long Term Investments \| Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		177
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		177
Long Term Investments \| U.S. Government Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		12,016
Gross Unrealized Gains		5
Gross Unrealized Losses		(16)
Fair Value		$ 12,005

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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