Fair Value Measurement and Fair Value of Financial Instruments - Reconciliation of Assets and Liabilities on a Recurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Trading Assets
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	$ 311	$ 271
Total gains (losses) (realized/unrealized) included in income before taxes, assets	(29)	61
Purchases/additions, assets	2	4
Settlements, assets	(56)	(25)
Asset balance, end of period	228	311
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, assets	(29)	61
Securities Available for Sale
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	1,790	2,018
Total gains (losses) (realized/unrealized) included in other comprehensive income, assets	1	29
Purchases/additions, assets	148	197
Settlements, assets	(324)	(454)
Asset balance, end of period	1,603	1,790
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(12)
Other assets
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	2	1
Total gains (losses) (realized/unrealized) included in income before taxes, assets	(1)	1
Asset balance, end of period	1	2
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, assets	(1)	1
Trading Liabilities
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liability balance, beginning of period	(305)	(264)
Total gains (losses) (realized/unrealized) included in income before taxes, assets, liability	27	(61)
Sales, liabilities	(2)	(5)
Settlements, liabilities	57	25
Liability balance, end of period	(223)	(305)
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, liabilities	27	(61)
Other Liabilities
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liability balance, beginning of period	(107)	(99)
Total gains (losses) (realized/unrealized) included in income before taxes, assets, liability	(7)	(8)
Liability balance, end of period	(114)	(107)
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, liabilities	$ (7)	$ (8)

Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those liabilities still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of sales of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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