Securities - Securities Available for Sale with a Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	$ 3,089	$ 4,677
Securities available for sale, Less than 12 months, Unrealized Losses	20	50
Securities available for sale, 12 months or more, Fair Value	9,584	9,295
Securities available for sale, 12 months or more, Unrealized Losses	231	357
Securities available for sale, Fair Value, Total	12,673	13,972
Securities available for sale, Unrealized Losses, Total	251	407
U.S. Treasury
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	0	147
Securities available for sale, Less than 12 months, Unrealized Losses	0	1
Securities available for sale, 12 months or more, Fair Value	3,136	3,182
Securities available for sale, 12 months or more, Unrealized Losses	91	143
Securities available for sale, Fair Value, Total	3,136	3,329
Securities available for sale, Unrealized Losses, Total	91	144
US Government Agencies Debt Securities [Member]
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	1
Securities available for sale, Less than 12 months, Unrealized Losses	0
Securities available for sale, 12 months or more, Fair Value	0
Securities available for sale, 12 months or more, Unrealized Losses	0
Securities available for sale, Fair Value, Total	1
Securities available for sale, Unrealized Losses, Total	0
U.S. government agency and government-sponsored agencies
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	1,527	1,941
Securities available for sale, Less than 12 months, Unrealized Losses	11	8
Securities available for sale, 12 months or more, Fair Value	4,877	4,797
Securities available for sale, 12 months or more, Unrealized Losses	97	160
Securities available for sale, Fair Value, Total	6,404	6,738
Securities available for sale, Unrealized Losses, Total	108	168
Privately issued
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	36	398
Securities available for sale, Less than 12 months, Unrealized Losses	0	7
Securities available for sale, 12 months or more, Fair Value	598	383
Securities available for sale, 12 months or more, Unrealized Losses	12	16
Securities available for sale, Fair Value, Total	634	781
Securities available for sale, Unrealized Losses, Total	12	23
Privately issued - commercial mortgage-backed securities
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	80	364
Securities available for sale, Less than 12 months, Unrealized Losses	0	6
Securities available for sale, 12 months or more, Fair Value	525	512
Securities available for sale, 12 months or more, Unrealized Losses	5	14
Securities available for sale, Fair Value, Total	605	876
Securities available for sale, Unrealized Losses, Total	5	20
Collateralized loan obligations
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	1,351	1,428
Securities available for sale, Less than 12 months, Unrealized Losses	8	18
Securities available for sale, 12 months or more, Fair Value	32	0
Securities available for sale, 12 months or more, Unrealized Losses	1	0
Securities available for sale, Fair Value, Total	1,383	1,428
Securities available for sale, Unrealized Losses, Total	9	18
Asset Liability Management securities
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	2,995	4,278
Securities available for sale, Less than 12 months, Unrealized Losses	19	40
Securities available for sale, 12 months or more, Fair Value	9,168	8,874
Securities available for sale, 12 months or more, Unrealized Losses	206	333
Securities available for sale, Fair Value, Total	12,163	13,152
Securities available for sale, Unrealized Losses, Total	225	373
Direct bank purchase bonds
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	19	221
Securities available for sale, Less than 12 months, Unrealized Losses	1	6
Securities available for sale, 12 months or more, Fair Value	412	417
Securities available for sale, 12 months or more, Unrealized Losses	25	24
Securities available for sale, Fair Value, Total	431	638
Securities available for sale, Unrealized Losses, Total	26	30
Other
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	75	178
Securities available for sale, Less than 12 months, Unrealized Losses	0	4
Securities available for sale, 12 months or more, Fair Value	4	4
Securities available for sale, 12 months or more, Unrealized Losses	0	0
Securities available for sale, Fair Value, Total	79	182
Securities available for sale, Unrealized Losses, Total	$ 0	$ 4

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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