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Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 37,638 | $ 36,551
Gross Unrealized Gains | 3 |
Gross Unrealized Losses | (3) | (2)
Estimated Fair Value | 37,638 | 36,549
Money Market Funds |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 14,334 | 10,156
Estimated Fair Value | 14,334 | 10,156
Money Market Funds | Restricted cash (money market funds) |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 5,330 | 19,593
Gross Unrealized Gains | 3 |
Estimated Fair Value | 5,333 | 19,593
U.S. Government Agency Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 16,474 | 6,802
Gross Unrealized Losses | (3) | (2)
Estimated Fair Value | 16,471 | 6,800
Cash and Cash Equivalents |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 14,334 | 10,156
Short-Term Investments |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Estimated Fair Value | 17,971 | $ 6,800
Agency Securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 1,500 |
Estimated Fair Value | $ 1,500 |

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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