Held-to-Maturity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	$ 27,182	$ 21,969
Held-to-maturity securities, gross unrecognized gains	950	328
Held-to-maturity securities, gross unrecognized losses	(36)	(51)
Held-to-maturity securities, fair value	28,096	22,246
Agency mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	25,579	20,085
Held-to-maturity securities, gross unrecognized gains	870	293
Held-to-maturity securities, gross unrecognized losses	(34)	(49)
Held-to-maturity securities, fair value	26,415	20,329
Agency debentures [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	208	267
Held-to-maturity securities, gross unrecognized gains	13	4
Held-to-maturity securities, gross unrecognized losses	(2)	0
Held-to-maturity securities, fair value	219	271
Other agency debt securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	1,395	1,617
Held-to-maturity securities, gross unrecognized gains	67	31
Held-to-maturity securities, gross unrecognized losses	0	(2)
Held-to-maturity securities, fair value	$ 1,462	$ 1,646

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=118954083&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=118939692&loc=d3e27232-111563

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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