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Investment Securities - Schedule of Investment Securities Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 272,331 | $ 365,324
Unrealized Gains | 1 | 172
Unrealized Losses | (408) | (80)
Estimated Fair Value | 271,924 | 365,416
U.S. Treasury notes |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 32,976 | 82,484
Unrealized Gains |  | 6
Unrealized Losses | (12) | (3)
Estimated Fair Value | 32,964 | 82,487
Government sponsored enterprise securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 10,082 | 73,789
Unrealized Gains |  | 1
Unrealized Losses | (10) | (5)
Estimated Fair Value | 10,072 | 73,785
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 138,650 | 79,190
Unrealized Gains | 1 |
Unrealized Losses | (321) | (72)
Estimated Fair Value | 138,330 | 79,118
Commercial paper |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 90,623 | 129,861
Unrealized Gains |  | 165
Unrealized Losses | (65) |
Estimated Fair Value | $ 90,558 | $ 130,026

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt 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 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 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 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 2: 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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- Definition Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer. + 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 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562 + Details
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