Investment securities (Investments Classified by Credit Rating) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 1,126	$ 1,253
Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,126	1,253
AA Rating
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,126	1,253
AA Rating \| Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 1,126	$ 1,253

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
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Name:	us-gaap_HeldToMaturitySecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant