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, 2016	Dec. 31, 2015
Trading Assets
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	$ 228	$ 311
Total gains (losses) (realized/unrealized) included in income before taxes, assets	31	(29)
Purchases/additions, assets		2
Settlements, assets	(91)	(56)
Asset balance, end of period	168	228
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, assets	31	(29)
Securities Available for Sale
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	1,603	1,790
Total gains (losses) (realized/unrealized) included in other comprehensive income, assets	(13)	1
Purchases/additions, assets	286	148
Settlements, assets	(238)	(324)
Transfers out of level 3, assets		(12)
Asset balance, end of period	1,638	1,603
Other Assets
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	1	2
Total gains (losses) (realized/unrealized) included in income before taxes, assets	3	(1)
Purchases/additions, assets	7
Transfers into level 3, assets	13
Asset balance, end of period	24	1
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, assets	3	(1)
Trading Liabilities
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liability balance, beginning of period	(223)	(305)
Total gains (losses) (realized/unrealized) included in income before taxes, assets, liability	(32)	27
Sales, liabilities		(2)
Settlements, liabilities	89	57
Liability balance, end of period	(166)	(223)
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, liabilities	(32)	27
Other Liabilities
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liability balance, beginning of period	(114)	(107)
Total gains (losses) (realized/unrealized) included in income before taxes, assets, liability	(7)	(7)
Liability balance, end of period	(121)	(114)
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, liabilities	$ (7)	$ (7)

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

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-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

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-Publisher FASB
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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.

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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
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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