Noninterest Income - Gains and Losses on Investments (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Income and Expenses [Abstract]
Total net gains on non-marketable and other equity securities	$ 32,193	$ 15,339	$ 77,365	$ 48,454
(Losses) gains on sales of available-for-sale securities, net	0	(101)	0	384
Gains on investment securities, net	$ 32,193	$ 15,238	$ 77,365	$ 48,838

Amount of realized and unrealized gain (loss) on investment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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