Quarterly report pursuant to Section 13 or 15(d)

Investments in Marketable Securities - Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Type of Security (Detail)

v3.19.1
Investments in Marketable Securities - Schedule of Amortized Cost and Fair Value of Marketable Securities, Available-for-Sale, by Type of Security (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 195,787 $ 221,454
Fair Value 196,103 220,645
Short-term investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 121,050 137,532
Gross Unrealized Gains 35 7
Gross Unrealized Losses (26) (103)
Fair Value 121,059 137,436
Short-term investments [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 104,049 121,252
Gross Unrealized Gains 35 7
Gross Unrealized Losses (10) (79)
Fair Value 104,074 121,180
Short-term investments [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 0 3,512
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 (7)
Fair Value 0 3,505
Short-term investments [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 16,196 11,962
Gross Unrealized Gains 0 0
Gross Unrealized Losses (12) (11)
Fair Value 16,184 11,951
Short-term investments [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 805 806
Gross Unrealized Gains 0 0
Gross Unrealized Losses (4) (6)
Fair Value 801 800
Long-term marketable securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 74,737 83,922
Gross Unrealized Gains 462 143
Gross Unrealized Losses (155) (856)
Fair Value 75,044 83,209
Long-term marketable securities [Member] | U.S. Treasuries [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 33,560 44,997
Gross Unrealized Gains 164 128
Gross Unrealized Losses (54) (115)
Fair Value 33,670 45,010
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,519 1,569
Gross Unrealized Gains 0 0
Gross Unrealized Losses (40) (62)
Fair Value 1,479 1,507
Long-term marketable securities [Member] | Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 33,996 32,467
Gross Unrealized Gains 264 3
Gross Unrealized Losses (55) (633)
Fair Value 34,205 31,837
Long-term marketable securities [Member] | Asset-backed securities and other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 5,662 4,889
Gross Unrealized Gains 34 12
Gross Unrealized Losses (6) (46)
Fair Value $ 5,690 $ 4,855