Investment Securities - Changes to Carrying Amount of Other Investments without Readily Determinable Fair Value (Detail) $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
Carrying value as of January 1, 2018	$ 27,680
Upward carrying value adjustments	4,854
Downward carrying value adjustments	(1,729)
Additions	3,870
Sales and dispositions	(9,422)
Carrying value as of September 30, 2018	$ 25,253

Equity Securities Without Readily Determinable Fair Value, Additions, Annual Amount

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- Definition

Equity Securities Without Readily Determinable Fair Value, Sales And Dispositions, Annual Amount

+ References

No definition available.

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- Definition

Amount of investment in equity security without readily determinable fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117539-209714

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- Definition

Amount of loss from downward price adjustment on investment in equity security without readily determinable fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117539-209714

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- Definition

Amount of gain from upward price adjustment on investment in equity security without readily determinable fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117539-209714

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