Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Securities borrowed or purchased under resale agreements	$ 23,943	$ 22,368
Securities held to maturity	9,508	10,720
Restricted Cash and Cash Equivalents	54	48	$ 136
Level 1
Assets
Cash and cash equivalents	9,641	8,350
Securities borrowed or purchased under resale agreements	0	0
Securities held to maturity	0	0
Loans held for investment	0	0
Restricted Cash and Cash Equivalents	54	48
Liabilities
Time deposits	0	0
Securities loaned or sold under repurchase agreements	0	0
Commercial paper and other short-term borrowings	0	0
Long-term debt	0	0
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	0	0
Level 2
Assets
Cash and cash equivalents	0	0
Securities borrowed or purchased under resale agreements	23,946	22,368
Securities held to maturity	9,508	10,720
Loans held for investment	0	0
Restricted Cash and Cash Equivalents	0	0
Liabilities
Time deposits	15,691	11,714
Securities loaned or sold under repurchase agreements	28,866	27,285
Commercial paper and other short-term borrowings	6,484	9,263
Long-term debt	17,344	17,961
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	0	0
Level 3
Assets
Cash and cash equivalents	0	0
Securities borrowed or purchased under resale agreements	0	0
Securities held to maturity	0	0
Loans held for investment	87,506	84,729
Restricted Cash and Cash Equivalents	0	0
Liabilities
Time deposits	0	0
Securities loaned or sold under repurchase agreements	0	0
Commercial paper and other short-term borrowings	0	0
Long-term debt	0	0
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	280	242
Fair Value
Assets
Cash and cash equivalents	9,641	8,350
Securities borrowed or purchased under resale agreements	23,946	22,368
Securities held to maturity	9,508	10,720
Loans held for investment	87,506	84,729
Other assets	54	48
Liabilities
Time deposits	15,691	11,714
Securities loaned or sold under repurchase agreements	28,866	27,285
Commercial paper and other short-term borrowings	6,484	9,263
Long-term debt	17,344	17,961
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	280	242
Carrying Amount
Assets
Cash and cash equivalents	9,641	8,350
Securities borrowed or purchased under resale agreements	23,943	22,368
Securities held to maturity	9,421	10,901
Loans held for investment	86,687	84,805
Restricted Cash and Cash Equivalents	54	48
Liabilities
Time deposits	15,651	11,739
Securities loaned or sold under repurchase agreements	28,866	27,285
Commercial paper and other short-term borrowings	6,484	9,263
Long-term debt	17,129	17,918
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	$ 87	$ 120

Loans held for investment, net of allowance for loan losses at fair value.

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Off-balance sheet instruments.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Fair value portion of other assets.

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Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount, after the effects of master netting arrangements, of funds outstanding loaned in the form of a security resale agreement between the entity and another party for the purchase and resale of identical or substantially the same securities at a date certain for a specified price. Includes purchases of participations in pools of securities that are subject to a resale agreement, assets not subject to a master netting arrangement and not elected to be offset.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

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Unused amount of commitments to extend credit to customers.

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