Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments(Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Assets
Securities held to maturity	$ 10,207	$ 8,412
Level 1
Assets
Cash and cash equivalents	4,529	5,751
Level 2
Assets
Securities held to maturity	10,207	8,412
Liabilities
Deposits	84,375	86,076
Commercial paper and other short-term borrowings	1,038	2,704
Long-term debt	12,351	7,073
Level 3
Assets
Loans held for investment, net of allowance for loan losses	77,640	77,324
Other assets	17	11
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	243	269
Carrying Amount
Assets
Cash and cash equivalents	4,529	5,751
Securities held to maturity	10,158	8,291
Loans held for investment, net of allowance for loan losses	76,150	75,475
Other assets	16	59
Liabilities
Deposits	84,340	86,004
Commercial paper and other short-term borrowings	1,038	2,704
Long-term debt	12,349	6,972
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	243	269
Fair Value
Assets
Cash and cash equivalents	4,529	5,751
Securities held to maturity	10,207	8,412
Loans held for investment, net of allowance for loan losses	77,640	77,324
Other assets	17	11
Liabilities
Deposits	84,375	86,076
Commercial paper and other short-term borrowings	1,038	2,704
Long-term debt	12,351	7,073
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	$ 243	$ 269

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

No definition available.

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- Definition

Off-balance sheet instruments.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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- Definition

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.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- References

No definition available.

+ Details

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- References

No definition available.

+ Details

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- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Fair value portion of other assets.

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Unused amount of commitments to extend credit to customers.

+ References

No definition available.

+ Details

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