Investment securities (Investments Classified by Credit Rating) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 1,255	$ 1,568
Debt Securities, held-to-maturity, less than 12 Months, fair value		283
Debt securities, held-to-maturity, unrealized loss, less than 12 Months		(6)
Debt Securities, held-to-maturity, 12 Months or Longer, fair value		3
Debt securities, held-to-maturity, unrealized loss, 12 months or longer		0
Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,249
Debt Securities, held-to-maturity, less than 12 Months, fair value		283
Debt securities, held-to-maturity, unrealized loss, less than 12 Months		(6)
Debt Securities, held-to-maturity, 12 Months or Longer, fair value		0
Debt securities, held-to-maturity, unrealized loss, 12 months or longer		0
Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	6
Debt Securities, held-to-maturity, less than 12 Months, fair value		0
Debt securities, held-to-maturity, unrealized loss, less than 12 Months		0
Debt Securities, held-to-maturity, 12 Months or Longer, fair value		3
Debt securities, held-to-maturity, unrealized loss, 12 months or longer		$ 0
AAA Rating
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	6
AAA Rating \| Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	0
AAA Rating \| Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	6
AA Rating
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,249
AA Rating \| Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,249
AA Rating \| Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 0

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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