Held-to-Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	$ 21,884	$ 23,839
Held-to-maturity securities, gross unrecognized gains	30	110
Held-to-maturity securities, gross unrecognized losses	(423)	(230)
Held-to-maturity securities, fair value	21,491	23,719
Agency mortgage-backed securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	18,085	20,502
Held-to-maturity securities, gross unrecognized gains	26	95
Held-to-maturity securities, gross unrecognized losses	(363)	(193)
Held-to-maturity securities, fair value	17,748	20,404
Agency Debentures [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	1,824	710
Held-to-maturity securities, gross unrecognized gains	0	0
Held-to-maturity securities, gross unrecognized losses	(16)	(2)
Held-to-maturity securities, fair value	1,808	708
Agency Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost	1,975	2,615
Held-to-maturity securities, gross unrecognized gains	4	15
Held-to-maturity securities, gross unrecognized losses	(44)	(35)
Held-to-maturity securities, fair value	$ 1,935	2,595
Other [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities, amortized cost		12
Held-to-maturity securities, fair value		$ 12

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=109979856&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=82897076&loc=d3e27232-111563

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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
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-URI http://asc.fasb.org/extlink&oid=82897076&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
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-Section 50
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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
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