Non-consolidated variable interest entities - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity
Principal Amount Outstanding on Loans Securitized or Asset-backed Financing Arrangement	$ 147	$ 63
FNMA, GNMA, FHLMC VIE
Variable Interest Entity
Principal Amount Outstanding on Loans Securitized or Asset-backed Financing Arrangement	$ 8,700	$ 9,900

This is the principal amount outstanding for securitized loans only (across all types of loans).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118948506&loc=d3e107314-111719

Name:	us-gaap_PrincipalAmountOutstandingOnLoansSecuritized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant