Investment Securities - Additional Information (Detail)	3 Months Ended
Mar. 31, 2021 USD ($) Security	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Investment Holdings [Line Items]
Gross realized gains(loss) on sale of investment securities	$ 0	$ 0
Gains (losses) on equity securities	$ 12,282,000	$ 20,782,000
Number of debt securities with aggregate gross unrealized losses \| Security	281
Unrealized losses on individual debt securities	$ (26,000,000)
Cost method equity securities	386,000,000
Available-for-Sale Securities [Member]
Investment Holdings [Line Items]
Allowance for credit losses	$ 0		$ 0

Number of debt securities with gross losses.

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Amount of unrealized and realized gain (loss) on investment in debt and equity securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

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Amount of realized gain (loss) on investment in debt and equity securities.

+ References

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Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

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-Publisher FASB
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Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

+ References

No definition available.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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