Variable Interest Entities - Re-securitizations (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Transfers of securities to VIEs
U.S. GSEs and government agencies	$ 46,123,000,000	$ 25,852,000,000	$ 15,532,000,000
Private label securities transferred to re-securitization VIEs	0	0	$ 0
U.S. GSEs and government agencies \| Nonconsolidated re-securitization VIEs
Variable Interest Entity [Line Items]
Interest in VIEs	$ 2,631,000,000	$ 2,928,000,000

Amount of net assets (liabilities).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 30
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77885760&loc=SL35686385-199418

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 205
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=118262037&loc=d3e9915-115836

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(7))
-URI http://asc.fasb.org/extlink&oid=120401555&loc=SL114874367-224272

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section S99
-Paragraph 2
-Subparagraph (SX 210.6-05(4))
-URI http://asc.fasb.org/extlink&oid=120401414&loc=d3e604008-122996

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 220
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-09(6))
-URI http://asc.fasb.org/extlink&oid=120401555&loc=SL114874367-224272

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