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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
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

X

- 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

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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)
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+ 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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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