Investment Securities - Additional Information (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)
Investment Holdings [Line Items]
Loss on bank investment securities	$ (3,415,000)		$ (10,520,000)	$ (17,000)
Gross realized gains(loss) on sale of investment securities	$ 0	$ 0	$ 0	$ 0
Number of debt securities with aggregate gross unrealized losses \| Security	1,503		1,503
Unrealized losses on individual debt securities			$ (425,000,000)
Cost method equity securities	$ 411,000,000		411,000,000
Retained Earnings [Member] \| Adjustments for New Accounting Pronouncement [Member]
Investment Holdings [Line Items]
Reclassification from accumulated comprehensive income to retained earnings, net of tax	$ 17,000,000		$ 17,000,000

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
-Subparagraph (SX 210.5-03(9)(a))
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Amount of realized gain (loss) on investment in debt and equity securities.

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No definition available.

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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.

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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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Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
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