Securities - Securities Available for Sale with a Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	$ 7,244	$ 5,561
Securities available for sale, Less than 12 months, Unrealized Losses	163	44
Securities available for sale, 12 months or more, Fair Value	6,156	7,586
Securities available for sale, 12 months or more, Unrealized Losses	346	245
Securities available for sale, Fair Value, Total	13,400	13,147
Securities available for sale, Unrealized Losses, Total	509	289
U.S. Treasury
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	1,289	1,074
Securities available for sale, Less than 12 months, Unrealized Losses	45	14
Securities available for sale, 12 months or more, Fair Value	2,078	2,128
Securities available for sale, 12 months or more, Unrealized Losses	158	104
Securities available for sale, Fair Value, Total	3,367	3,202
Securities available for sale, Unrealized Losses, Total	203	118
U.S. government agency and government-sponsored agencies
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	3,789	3,606
Securities available for sale, Less than 12 months, Unrealized Losses	68	22
Securities available for sale, 12 months or more, Fair Value	3,378	4,651
Securities available for sale, 12 months or more, Unrealized Losses	147	110
Securities available for sale, Fair Value, Total	7,167	8,257
Securities available for sale, Unrealized Losses, Total	215	132
Privately issued
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	600	275
Securities available for sale, Less than 12 months, Unrealized Losses	16	1
Securities available for sale, 12 months or more, Fair Value	152	164
Securities available for sale, 12 months or more, Unrealized Losses	7	3
Securities available for sale, Fair Value, Total	752	439
Securities available for sale, Unrealized Losses, Total	23	4
Privately issued - commercial mortgage-backed securities
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	800	447
Securities available for sale, Less than 12 months, Unrealized Losses	23	3
Securities available for sale, 12 months or more, Fair Value	99	80
Securities available for sale, 12 months or more, Unrealized Losses	5	2
Securities available for sale, Fair Value, Total	899	527
Securities available for sale, Unrealized Losses, Total	28	5
Collateralized loan obligations
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	415	12
Securities available for sale, Less than 12 months, Unrealized Losses	1	0
Securities available for sale, 12 months or more, Fair Value	0	0
Securities available for sale, 12 months or more, Unrealized Losses	0	0
Securities available for sale, Fair Value, Total	415	12
Securities available for sale, Unrealized Losses, Total	1	0
Asset Liability Management securities
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	6,893	5,414
Securities available for sale, Less than 12 months, Unrealized Losses	153	40
Securities available for sale, 12 months or more, Fair Value	5,707	7,023
Securities available for sale, 12 months or more, Unrealized Losses	317	219
Securities available for sale, Fair Value, Total	12,600	12,437
Securities available for sale, Unrealized Losses, Total	470	259
Direct bank purchase bonds
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	329	58
Securities available for sale, Less than 12 months, Unrealized Losses	10	4
Securities available for sale, 12 months or more, Fair Value	449	563
Securities available for sale, 12 months or more, Unrealized Losses	29	26
Securities available for sale, Fair Value, Total	778	621
Securities available for sale, Unrealized Losses, Total	39	30
Other
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value	22	79
Securities available for sale, Less than 12 months, Unrealized Losses	0	0
Securities available for sale, 12 months or more, Fair Value	0	0
Securities available for sale, 12 months or more, Unrealized Losses	0	0
Securities available for sale, Fair Value, Total	22	79
Securities available for sale, Unrealized Losses, Total	$ 0	0
Equity securities
Securities available for sale
Securities available for sale, Less than 12 months, Fair Value		10
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		10
Securities available for sale, Unrealized Losses, Total		$ 0

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
-SubTopic 10
-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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- Definition

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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+ References

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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