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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 | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 193,623 | $ 221,454
Fair Value | 195,260 | 220,645
Short-term investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 124,227 | 137,532
Gross Unrealized Gains | 249 | 7
Gross Unrealized Losses | (1) | (103)
Fair Value | 124,475 | 137,436
Short-term investments [Member] | U.S. Treasuries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 109,208 | 121,252
Gross Unrealized Gains | 192 | 7
Gross Unrealized Losses | (1) | (79)
Fair Value | 109,399 | 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 | 15,019 | 11,962
Gross Unrealized Gains | 57 | 0
Gross Unrealized Losses | 0 | (11)
Fair Value | 15,076 | 11,951
Short-term investments [Member] | Asset-backed securities and other [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 0 | 806
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | (6)
Fair Value | 0 | 800
Long-term marketable securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 69,396 | 83,922
Gross Unrealized Gains | 1,420 | 143
Gross Unrealized Losses | (31) | (856)
Fair Value | 70,785 | 83,209
Long-term marketable securities [Member] | U.S. Treasuries [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 30,146 | 44,997
Gross Unrealized Gains | 325 | 128
Gross Unrealized Losses | (15) | (115)
Fair Value | 30,456 | 45,010
Long-term marketable securities [Member] | U.S. Government Sponsored Entities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 1,415 | 1,569
Gross Unrealized Gains | 2 | 0
Gross Unrealized Losses | (4) | (62)
Fair Value | 1,413 | 1,507
Long-term marketable securities [Member] | Corporate debt securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 29,870 | 32,467
Gross Unrealized Gains | 980 | 3
Gross Unrealized Losses | (8) | (633)
Fair Value | 30,842 | 31,837
Long-term marketable securities [Member] | Asset-backed securities and other [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 7,965 | 4,889
Gross Unrealized Gains | 113 | 12
Gross Unrealized Losses | (4) | (46)
Fair Value | $ 8,074 | $ 4,855

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454 + Details
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