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, 2018	Dec. 31, 2017
Trading Assets
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	$ 139	$ 168
Total gains (losses) (realized/unrealized) included in income before taxes, assets	(10)	73
Settlements, assets	(116)	(102)
Asset balance, end of period	13	139
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, assets	(10)	73
Securities Available for Sale
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	1,600	1,638
Total gains (losses) (realized/unrealized) included in other comprehensive income, assets	(9)	(3)
Purchases/additions, assets	126	83
Settlements, assets	(386)	(118)
Asset balance, end of period	1,331	1,600
Other Assets
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	65	24
Total gains (losses) (realized/unrealized) included in income before taxes, assets	(8)	(3)
Purchases/additions, assets	220	44
Transfers into level 3, assets	0
Asset balance, end of period	277	65
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, assets	(8)	(3)
Trading Liabilities
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liability balance, beginning of period	(138)	(166)
Total gains (losses) (realized/unrealized) included in income before taxes, assets, liability	9	(72)
Settlements, liabilities	116	100
Liability balance, end of period	(13)	(138)
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, liabilities	9	(72)
Other Liabilities
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liability balance, beginning of period	(119)	(121)
Total gains (losses) (realized/unrealized) included in income before taxes, assets, liability	1	2
Liability balance, end of period	(118)	(119)
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, liabilities	1	$ 2
Fair Value, Measurements, Nonrecurring [Member]
Reconciliation of assets measured using significant unobservable inputs
Renewable energy investments	11
Fair Value, Measurements, Nonrecurring [Member] \| Level 3
Reconciliation of assets measured using significant unobservable inputs
Renewable energy investments	$ 11

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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 assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

+ 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 (d)
-URI http://asc.fasb.org/extlink&oid=66048111&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.

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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.

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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.

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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.

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

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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.

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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.

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