Available-for-Sale Securities (Contractual Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 16,603	$ 18,932
Fair Value	16,651	19,019
Non-Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Contractual Maturities, Due in One Year or Less, Amortized Cost	92	74
Contractual Maturities, Due in One Year or Less, Fair Value	92	74
Contractual Maturities, Due after One Year through Five Years, Amortized Cost	2,099	1,314
Contractual Maturities, Due after One Year through Five Years, Fair Value	2,110	1,323
Contractual Maturities, Due after Five Years through Ten Years, Amortized Cost	1,294	2,497
Contractual Maturities, Due after Five Years through Ten Years, Fair Value	1,302	2,506
Contractual Maturities, Due after Ten Years, Amortized Cost	708	808
Contractual Maturities, Due after Ten Years, Fair Value	724	826
Amortized Cost	4,193	4,693
Fair Value	4,228	4,729
Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,410	14,239
Fair Value	$ 12,423	$ 14,290

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: 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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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