The amount deducted from an entities' taxes, reflecting a percentage of an amount invested in certain assets spread over the useful life of those assets; includes deferred investment tax credits.

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Amount of financial support provided to a nonconsolidated legal entity that is required to comply with or operate in accordance with requirements that are similar to those included in Rule 2a-7 of the Investment Company Act of 1940 for registered money market funds.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

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The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position.

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The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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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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Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

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Net carrying amount of the assets and liabilities in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), when the reporting entity is not the VIE's primary beneficiary.

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Expenses related to investments.

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Expenses Related To Investments Recognized In Tax Expense

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Federal and state income tax credits recognized in tax expense.

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Accounting for Transfers and Servicing of Financial Assets and Variable Interest Entities - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Minimum [Member]
Variable Interest Entity [Line Items]
Aggregate amount of investments in unconsolidated VIEs	$ 1,000,000		$ 1,000,000		$ 1,000,000
Maximum [Member]
Variable Interest Entity [Line Items]
Aggregate amount of investments in unconsolidated VIEs	45,000,000		45,000,000		56,000,000
Financial Support Waived Fees [Member]
Variable Interest Entity [Line Items]
Financial or other support to money market funds	6,000,000	$ 6,000,000	18,000,000	$ 17,000,000
Private Investment Funds and Partnerships [Member]
Variable Interest Entity [Line Items]
Investment carrying amount	30,000,000		30,000,000		30,000,000
Maximum exposure to loss	55,000,000		55,000,000		51,000,000
Community Development and Tax Advantaged Investments [Member]
Variable Interest Entity [Line Items]
Federal and state income tax credits recognized in tax expense	150,000,000	173,000,000	486,000,000	495,000,000
Expense related to investments	133,000,000	163,000,000	434,000,000	464,000,000
Investment tax credits	90,000,000	361,000,000	368,000,000	843,000,000
Expenses related to investments recognized in tax expense	67,000,000	$ 61,000,000	202,000,000	$ 187,000,000
Investment carrying amount	5,517,000,000		5,517,000,000		5,660,000,000
Maximum exposure to loss	12,173,000,000		12,173,000,000		12,120,000,000
Assets related to consolidated VIEs	4,000,000,000		4,000,000,000		3,500,000,000
Liabilities related to consolidated VIEs	2,900,000,000		2,900,000,000		2,500,000,000
Conduit [Member]
Variable Interest Entity [Line Items]
Assets related to consolidated VIEs	18,000,000		18,000,000		18,000,000
Tender Option Bond Program [Member]
Variable Interest Entity [Line Items]
Assets related to consolidated VIEs	2,400,000,000		2,400,000,000		2,500,000,000
Liabilities related to consolidated VIEs	$ 2,300,000,000		$ 2,300,000,000		$ 2,300,000,000